Synthesis, Characterization and Assembly of the Binary Ligand Protected Gold Nanoparticles by
Hyewon Kim
M.S. Materials Science and Engineering
Seoul National University, 2005
B.S. Materials Science and Engineering
Seoul National University, 2003
__________
MASSACHUSETTS INS1TTUTE
OF TECHNOLOGY
MAY 14 2014
LBRARIES
Submitted to the Department of Materials Science and Engineering in Partial Fulfillment of the Requirements for the Degree of
Doctor of Philosophy at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
September 2013
0 2013 Massachusetts Institute of Technology.
All rights reserved
Signature of Author
De, rtment of Materials Science and Engineering
August 8, 2013
Certified by
Francesco Stellacci
Adjunct Associate Professor of Materials Science and Engineering
Thesis Supervisor
Accepted by<
Gerbrand Ceder
Professor of Department of Materials Science and Engineering
Chair, Departmental Committee on Graduate Students
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2

Synthesis, Characterization and Assembly of the Binary Ligand Protected Gold Nanoparticles by
Hyewon Kim
Submitted to the Department of Materials Science and Engineering on August 8, 2013 in
Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in
Materials Science and Engineering
When a gold nanoparticle is coated with two dislike ligands, the ligands selfassemble on the nanoparticle surface and the phase separation occurs based on the miscibility and the size mismatch of two ligands, and the sizes of nanoparticles. When the size of the gold core is approximately between 3-8 nm, the stripe-like ordered domains of two ligands are formed. The stripe-like structure is not favored when you consider only the enthalpy. However, the long ligands obtain extra free-volumes when they are surrounded by the short ligands due to the curvature of a nanoparticle, hence, the entropy increases when two ligands are mixed on the nanoparticle surface. The balance between enthalpy and entropy leads to the state where the stripe-like arrangement of two ligands is thermodynamically the most stable.
When the size of the gold core becomes smaller, the entropy contribution becomes less and less relevant, since the gain of free-volume when two different ligands are closely placed is smaller due to the larger curvature of smaller nanoparticles. Under this condition, the final morphology is primarily determined by the enthalpy of separation.
Therefore, for small particles, two ligands phase separate into two bulk phases, resulting the Janus nanoparticles. In the first part of this thesis, we demonstrate that gold nanoparticles with a core diameter smaller than 1.5 nm form Janus nanoparticles in many ligand combinations. We used four different nanoparticles and different techniques to confirm the presence of a majority of Janus particles. All of them show similar cut-off sizes for the Janus-to-stripe transition.
In the second part of this thesis, we show nanoparticle hydrogels using the selfassembly of the stripe nanoparticles. One of unique surface properties of the stripe nanoparticle is divalency. A particle coated with stripe-like domains implies two defect points at the poles of NPs. These two polar defects can be selectively functionalized with molecules that in turn can act as handles for further assemblies. The network structure is formed only using ionic interaction between NPs, and it requires both divalent anionic nanoparticles and divalent cations. Gels are investigated to determine their properties using rheological characterization.
Thesis Advisor: Francesco Stellacci
Title: Adjunct Associate Professor of Materials Science and Engineering
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First of all, I would like to thank my advisor, Prof. Francesco Stellacci, for inspiring and supporting me throughout my graduate life at MIT. It has been pleasure to learn from him and work with him. I also would like to sincerely thank Prof. Michael
Rubner for his encouragement and support for my work and for being my second advisor at MIT for the last few years of my study. I thank Prof. Patrick Doyle for his insightful advice on my research.
I appreciate all my collaborators, Randy Carney, Javier Reguera, Quy Ong, Xiang
Liu, Samuel Jones, Eric Appel and Prof. Oren Scherman for their help on experiments and insightful discussions. I thank all my past and current SuNMag/SuNMIL group memebers for their friendship. I also thank Cohen-Rubner group members for being a supportive and constructive group members. I am also blessed to have a great Korean community at MIT. I would like to appreciate their warm friendship that helped me overcome many difficulties.
Last but certainly not least, I truly thank my husband, Jun, for his love, encouragement and support. Without him, I would not have done this work. I also want to thank my son, Daniel, who has learned to understand busy mom. I appreciate all my family members, my parents, my brother, my parents-in-law and sister-in-law for their ongoing support and love.
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A bstract
L ist of Figures ................................................................................................................
C hapter 1. Introduction .......................................................................................
1.1 M otivation...............................................................................................................................
1.2. Overview and Scope ..............................................................................................................
1.3. References ..............................................................................................................................
11
16
18
3
Chapter 2. Ligand Arrangement in Binary Ligand Nanoparticles..........22
2.1. Self- assembled M onolayer (SAM )................................................................................. 22
2.2. Phase Separation of Binary Ligand on Surface ............................................................ 23
2.2.1. Phase Separation of Binary Ligand on Flat Surface .................................... 23
2.1.2 Phase Separation of Binary Ligand on Gold Nanoparticles ........................ 25
2.2. Properties of the Stripe Gold Nanoparticles................................................................... 28
2.3. Divalency of the Striped Gold Nanoparticles ................................................................ 29
2.4. Place-exchange of Ligands on Gold Nanoparticles ........................................................ 30
2.5. Nanoparticle Chains using Divalent Gold Nanoparticles............................................. 31
2.5.1. Synthesis of NT/M BT Gold Nanoparticles ................................................. 32
2.5.2. Pole-Functionalization of Gold Nanoparticles ............................................ 33
2.5.3. Polym erization of Divalent Gold Nanoparticles......................................... 33
2.5.4. Results..............................................................................................................34
2.5. References .............................................................................................................................. 36
5

A bstract......................................................................................................................................... 37
3.1. Introduction ........................................................................................................................... 37
3.1.1. Synthesis of Inorganic Janus Nanoparticles using Solid Template ............. 38
3.1.2. Synthesis of Inorganic Janus Nanoparticles using liquid-air or liquid-liquid interface ..................................................................................................................... 4 2
3.1.3. Synthesis of Inorganic Janus Nanoparticles using self-assembly of ligands...43
3.2. Synthesis of Janus Monolayer Protected Gold Nanoparticles ..................................... 45
3.2.1. Synthesis of HDT/TPT covered gold nanoparticles .................................... 46
3.2.2. Synthesis of TDT/AUDT covered gold nanoparticles.................................47
3.2.3. Synthesis of MPSA/TMA covered gold nanoparticles................................47
3.2.4. Synthesis of OT/MPSA covered gold nanoparticles ................................... 47
3.3. Characterization of Janus Monolayer Protected Gold Nanoparticles.........................48
3.3.1. Nuclear Overhauser Enhancement Spectroscopy (NOESY) of HDT/TPT gold nanoparticles .............................................................................................................. 48
3.3.2. Dimerization of TDT/AUDT gold nanoparticles ........................................ 50
3.3.3. Gel electrophoresis (GEP) of MPSA/TMA gold nanoparticles .................. 53
3.3.4. Self-assembly of OT/MPSA gold nanoparticles......................................... 55
3.4. Size Limit for Janus Gold Nanoparticles ........................................................................
3.5. C onclusions ............................................................................................................................
3.6. Experimental Section ........................................................................................................
3.6.1. M aterials .....................................................................................................
3.6.2. Synthesis of gold nanoparticles ....................................................................
57
58
59
59
59
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3.6.3. N OESY ............................................................................................................
3.6.4. Dim erization of TDT/AUDT NPs ...............................................................
3.6.5. AUC .................................................................................................................
3.6.6. TEM .................................................................................................................
3.6.7. Gel electrophoresis ......................................................................................
3.7. References ..............................................................................................................................
62
62
63
61
61
64
Chapter 4. The Functional Assembly of the Striped Nanoparticles:
N anoparticle G els.............................................................................................
Abstract .........................................................................................................................................
69
4.1. Introduction ...........................................................................................................................
70
4.1.1. Definition and the General Properties of Gels............................................. 70
4.1.2. Physical Gels using Polymeric M aterials .................................................... 71
4.1.3. Physical Gels using Inorganic/M etallic M aterials...................................... 75
4.2. Theoretical Background of Nanoparticle Gel.................................................................
4.3. Preliminary W ork .................................................................................................................
78
4.4. Design of the Divalent Nanoparticle Hydrogel...............................................................
4.5. Results ....................................................................................................................................
81
82
85
4.6. The Rheological Properties of the Nanoparticle Gel......................................................
4.7. Phase Diagram of Patchy Particle Gel ................................................................................
90
92
4.8. Conclusions and Outlook................................................................................................. 94
4.9. Experimental Section ........................................................................................................ 94
4.9.1. M aterials ...................................................................................................... 94
4.9.2. Synthesis of gold nanoparticles .................................................................... 95
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4.9.3. Pole-functionalization of gold nanoparticles............................................... 96
4.9.4. The G elation of gold nanoparticles.............................................................. 96
4.9.5. TEM of G old N anoparticles ............................................................................ 97
4.9.6. The Rheology of N anoparticle Gels ........................................................... 97
4.9. References .............................................................................................................................. 98
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Figure 1. 1 Schematic illustrations of anisotropic particles. ................................................. 12
Figure 1. 2 Model spherical patchy particles ........................................................................... 14
Figure 2.1 Binary ligand arrangement on the flat surface and the nanoparticle surface ..
24
Figure 2.2 Schematic drawing illustrating the conformational entropy for ligands on NPs..
.........................................................................................................................................................................
2 7
Figure 2. 3 Interfacial energy and solubility of the stripe nanoparticles ......................... 29
Figure 2. 4 1-D assembly of the stripe nanoparticles .............................................................. 32
Figure 2. 5 The effect of ligand ratio on the polymerization of the striped NPs. .......... 35
Figure 3. 1 Solid template based synthesis of Janus NPs using amine terminated glass slid e s ............................................................................................................................................................. 4 1
Figure 3. 2 Solid template based synthesis of Janus NPs using polymer single crystals .42
Figure 3. 3 Solid template based synthesis of Janus NPs using particle surfaces ...... 43
Figure 3. 4 Interface based synthesis of Janus NPs ................................................................
45
Figure 3.5 NOESY measurements of HDT/TPT mixed ligand coated nanoparticles. .... 51
Figure 3. 6 Dimerization scheme of TDT/AUDT Janus nanoparticles..............................54
Figure 3.7 Gel electrophoresis of TMA/MPSA Janus NPs....................................................56
Figure 3.8 TEM image of separated MPSA/TMA NPs .............................................................. 57
Figure 3. 9 Dimerization scheme of OT/MPSA Janus nanoparticles ................................ 58
Figure 3. 10 Pickering emulsion of Janus nanoparticles in toluene/water interface .......... 59
Figure 4. 1 The schematic graph represents the mechanical property of a generic g e ls.................................................................................................................................................................7
Figure 4. 2 The hydrogel made with clay nanosheets and dendritic binder ........... 73
0
Figure 4. 3 Supramolecular hydrogel using CB[8] ............................................................. 74
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Figure 4. 4 The formation of graphene oxide/iron oxide hydrogels in different pH co n d itio n .................................................................................................................................................... 7 6
Figure 4. 5 CdTe nanoparticle hydrogel/aerogel using 5-mercaptomethyltetrazole
.........................................................................................................................................................................
7 7
Figure 4. 6 The theoretical calculation of nanoparticle gel ........................................... 79
Figure 4. 7 The phase diagram of the mixture of divalent and trivalent particles. ....
80
Figure 4. 8 Nanoparticle gel using NT/MHol nanoparticles. ......................................... 82
Figure 4. 9 The gelation of the HT/EG3 striped NPs......................................................... 84
Figure 4. 10 The hydrogel formed by the striped NPs ..................................................... 86
Figure 4. 11 Control experim ents ....................................................................................................
88
Figure 4. 12 The rheology measurements of NP gels ....................................................... 91
Figure 4. 13 Reconstructed phase-diagram of the divalent NP gels.......93
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Nanoparticles (NPs) have drawn tremendous interest as a new building block in supramolecular chemistry. They are in sizes of 1- 100 nm, and they consist of the core materials covered with one or more organic ligands that stabilize the core and determine surface properties of the NPs. NPs in this size range have unique properties to NPs because the surface-to-volume ratio of NPs is dramatically increased compared to that of bulk materials. Whereas, NPs that are larger than 100 nm show the property of bulk materials, and NPs that are smaller than I nm behave more like a molecule or an artificial atom. Recent improvements of the synthesis methods enabled to create various functional nanoparticles using metals (e.g. gold, silver, platinum),' 4 semiconductors (e.g. cadmium selenide),j 6 oxides (e.g. titanium oxide, zinc oxide),'' and polymers 2
(e.g. polystyrene).
Significant amount of papers are available controlling sizes, 9 shapes,
10 and surface properties"'
12 of NPs.
Assembled NPs show the collective properties that are very different from those of the bulk material as well as those of individual NPs. New collective properties, such as the surface plasmon coupling,''
14 the exciton coupling, 1 5 '
16 and the plasmon-exciton interaction
17-19 lead to the applications in optoelectronics, 20 sensors,1
8
'21,22 and data control the spacing and alignment of individual NPs. Therefore, the study to control the assembly of the NPs is equally important as developing a new synthesis method of NPs.
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Self-assembly seems to be the most promising way to achieve the controlled clusters of NPs. There are many reviews on controlling self-assembly interaction between
NPs.2- Especially, the assembly using anisotropic NPs has brought great attention.,
Nanoscale forces that are used for self-assembly of NPs has been studied, and a comprehensive review states that these forces may differ from those for bulk materials.
30
Anisotropic particles, particles with broken symmetry, have drawn a lot of interest since they provide asymmetric surface properties and chemical functionalities. These properties enable the more complex assemblies of anisotropic NPs than those of isotropic
NPs. Anisotropic particles are generally classified into three different types; 1) NPs with surface anisotropy; 2) NPs with geometric anisotropy; 3) NPs with different core materials. (Figure 1.1)
(c)
Figure 1. 1 Schematic illustrations of anisotropic particles. Particles (a) with surface anisotropy (patchy particles), (b) with geometric anisotropy (snowman or dumbbell shape particles), and (c) with two or more different core materials (multi-component particles)
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Several different approaches were investigated to synthesize anisotropic particles.
3 1 ' 3 2 The most widely studied method is template-based or interface-based synthesis, which place particles between two interfaces or on a template, and selectively functionalize the one side of the particle and achieve anisotropic properties. A second method is to make the particles with asymmetric shape (i.e. dumbbell, snowman) or with asymmetric core (i.e. gold on TiO
2
) that usually involves two-step growth of two different particles. The last method is to use the self-assembly and the phase separation of two or more ligands that covers the core material and induce surface anisotropy.
Among the examples of anisotropic particles with various geometries shown in
Figure 1.1, the "patchy" particles are the mostly studied (Figure 1.1(a)).' 2 The patchy particles are defined as patterned spherical particles that have one or more discrete sites
("patches") on their surfaces. These patchy sites work as interaction sites and provide strong anisotropy on the particles; they enable directional interaction between particles or between particles and surfaces. These patchy particles are widely found in biological molecules, e.g. globular proteins, and pollen grains. Glotzer et. al. studied self-assembly of spherical patchy particles with various patchy geometries which are shown in Figure
1.2.12
13

i e
S f b C d g
h j k 1
Figure 1. 2 Model spherical patchy particles studied in ref.
12.
(reprinted from ref.
12)
Janus particles are a special class of patchy particles, which is named after the
Roman god with two faces. They have either two different surfaces with the same core
33
39, or two different core materials together.
4 0
'
4
' Many synthetic methods have developed for the past decade. The majority of methods used the step-wise modification of particles at interfaces. 3
4-16,39 To gain more control over surface functionality and an access to high purity of Janus particles still remain as a challenge in developing a new synthetic strategy.
The researchers in the Supramolecular Nanomaterials and Interfaces Group
(SuNMIL) have been studying the phase separation of two different ligands on the gold
NPs.
3 3
, 4 2 Previously, our group has found that two different ligands phase-separate in
14

alternating patterns, which forms the stripe-like structure on the surface of gold NPs. We also found that these stripe patterns exist in a certain core size range and certain ligand combinations.
2
' 43 This can be classified as one type of the patchy NPs with nonmonotonic solubility
44
, non-monotonic interfacial energy 45
, enhanced cell permeability 46
, catalytic activity 47 and molecular recognition 4 8
.'49.
Our group also showed 1-dimensional assembly of the striped NPs using selective pole-functionalization.5
0
,5
In this thesis, the synthesis, characterization, and assembly of the patchy gold NPs are presented. First, Janus NPs were synthesized using self-assembly of ligands. This new synthetic method is the direct pathway to yield Janus NPs without further modification. We showed this method works for various ligand combinations, and Janus
NPs exist in a certain size range. Second, the 3-dimensional assembly of the stripe NPs was investigated. This approach leads to a gelation of NPs via ionic interaction between patchy gold NPs.
is

This thesis comprises of two parts: a new synthetic method for Janus gold NPs and a new self-assembly method of the striped NPs that leads to functional materials.
In Chapter 2, the arrangements of two different ligands on the gold NPs surfaces are elucidated. Their different phase separation behaviors compared to those of flat surfaces, and the different phase evolution due to the size of the gold NPs are described.
The recent theoretical and experimental studies that confirm these arrangements are covered. From our previous study, we found that the stripe-like ligand arrangement is present in a certain size range of gold NPs with proper ligand combinations. We investigated the distinguished properties of these striped NPs. The details of these properties are also described. Among these properties, the most relevant property for this work is divalency of NPs. In this chapter, 1-dimensional assembly of the striped NPs by creating divalent NPs is explained.
Among the ligand arrangements that described in Chapter 2, the synthesis and characterization of Janus gold NPs are described in Chapter 3. The method that we developed in this study uses self-assembly and the phase-separation of two ligand molecules on the NPs' surfaces. This is the most straightforward method up to date. This method does not need a template, or further modification to achieve Janus structure, and we have shown the existence in various ligand combinations showing its generality and wide applicability. They are also the smallest Janus gold NPs currently available.
In Chapter 4, we used the. striped NPs and applied the assembly method developed in Chapter 2 to obtain the 3-dimensional functional assembly of the gold NPs.
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We created a hydrogel that consists of the water-soluble striped NPs. The gelation used ionic interactions between the striped NPs and divalent metal cations. These gels are easily made by changing the concentration of NPs. The gel properties are determined using rheology measurements This gelation method is unique because it uses the specific interaction sites on the NPs to generate self-assembled 3-D network structure. The structure utilizes self-assembly process requiring no additional chemicals. This concept also can be expanded to generate many different 3-D assemblies of gold NPs, and the gel properties that may be tunable for specific application.
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49. Cho, E. S.; Kim, J.; Tejerina, B.; Hermans, T. M.; Jiang, H.; Nakanishi, H.; Yu,
M.; Patashinski, A. Z.; Glotzer, S. C.; Stellacci, F.; Grzybowski, B. A., Ultrasensitive detection of toxic cations through changes in the tunnelling current across films of striped nanoparticles. Nature Materials 2012, 11 (11), 978-985.
50. DeVries, G. A.; Brunnbauer, M.; Hu, Y.; Jackson, A. M.; Long, B.; Neltner, B. T.;
Uzun, 0.; Wunsch, B. H.; Stellacci, F., Divalent metal nanoparticles. Science 2007, 315
(5810), 358-361.
51. DeVries, G. A.; Talley, F. R.; Carney, R. P.; Stellacci, F., Thermodynamic Study of the Reactivity of the Two Topological Point Defects Present in Mixed Self-Assembled
Monolayers on Gold Nanoparticles. Advanced Materials 2008, 20 (22), 4243-4247.
21

When ligand molecules adsorb onto a flat gold surface from solution or the gas phase, an ordered array of the ligand molecules is spontaneously formed into crystalline structure. The array is called a self-assembled monolayer (SAM). Molecules in a SAM consist of a headgroup, a backbone, and a terminal group. The headgroup binds to the substrate and make the ligand molecule chemically adsorb on the surface. In this work, we used thiolated molecules as ligands which contain a thiol group as a headgroup that binds to noble metal surfaces such as silver and gold. The backbones of the same ligands closely pack together in order to maximize the van der Waals interaction. In addition, they work as a physical barrier to the surface and determine the thickness of a SAM. The terminal functional group of ligand molecules controls the surface properties and provides chemical functionality to the surface.
The ligand molecules on the surface conforms to the same tilt angle and direction relative to the surface normal in order to maximize van der Waals interactions between each other.' For example, the tilt angle of alkanethiols on the flat gold surface is approximately 30 degrees, and in case of other ligands the tilt angles are different. It implies that when two different ligands are adsorbed on the surface, the complete mixed array of them on the surface would not be energetically favorable since they cannot accommodate their optimum tilt angles when two different molecules are close by. In addition, at defects or domain boundaries of a SAM, ligand molecules cannot
22

accommodate their tilt angle, thus molecules at defects would be less stabilized than those in the ordered crystalline phase of a SAM.
2.2. Phase Separation of Binary Ligand on Surface
2.2.1. Phase Separation of Binary Ligand on Flat Surface
When two mixed ligands adsorb onto a flat gold surface, they undergo a phase separation.
2-6 They phase-separate into either discrete molecular domains in nanometer scale 2 5 , or worm-like stripe domains.
6
The phase-separation occurs in various combinations of ligands when there is a size mismatch or different terminal functionalities in two different ligand molecules.
The worm-like stripe domains were found by using atomistic molecular dynamics simulation 6 as well as atomic force microscopy (AFM).
7 (Figure 2.1(a)) Glotzer and coworkers have found that the worm-like domains are a result of competition between enthalphy and entropy of phase-separation. Enthalpically, the bulk phase separation is favored. That is, the same molecules tend to pack together to make a single domain which lowers the enthalpy of the system. This enables molecules to maximize the intermolecular interaction with each other. For example, when the same molecules are packed, they conform their optimum tilt angle that maximizes van der Waals interaction, and sometimes they can form hydrogen bonds with the hydrophilic endgroups. On the other hand, randomly mixed structure of two ligands increases entropy, thus, when considering only entropy, different ligand molecules should arrange closely instead of phase-separation of two different ligand molecules. The ligand arrangement that balance the enthalpy and entropy can be illustrated as the same ligand molecules are closely
23

packed in some regions while different ligands are mixed together and create interfaces in the other regions. Therefore, nanometer sized worm-like domains are found as thermodynamically the most stable structure.
Topography Phase
(a)
4A
<U
(b) (c)
Figure 2.1 Binary ligand arrangement on the flat surface and the nanoparticle surface (a)
Self-assembled Monolayer (SAM) on the flat surface. The schematic cartoon (left), atomistic simulation result (middle), and atomic force microscopy (AFM) result (right) are shown.
Pictures are taken from ref 6 and 7. (b),(c) Scanning tunneling microscopy (STM) images of gold NPs. Gold NPs coated with decanethiol (DT)/mercaptopropionic acid (MPA) in composition 2:1 shows the striped structure (b), but gold NPs coated with octanethiol
(OT)/MPA in composition of 10:1 (c) shows packed and phase separated domains instead.
(Reproduced from ref. 8)
24

2.1.2 Phase Separation of Binary Ligand on Gold Nanoparticles
Thiolated molecules also form a SAM on gold NPs. This monolayer stabilizes
NPs from aggregation, determines the surface properties of NPs, and provides surface functionalities to NPs. When two different ligands adsorb on a gold NPs surface, the phase-separation occurs, but its behavior differs from that of the worm-like domain on a flat substrate due to the NPs' curvature.
6
There are two reasons that these two behaviors are different. First, the domain size of the phase-separated SAM is often the similar to that of NPs. Second, the spherical shape of NPs gives boundary conditions on molecular arrangement.
When two dissimilar ligands are self-assembled on gold NPs, they phase separate based on their miscibility, their size mismatch, and the NPs' sizes.
6
Previously, our group has shown a SAM of two immiscible ligands on gold NPs using scanning tunneling microscopy (STM) and found a nanometer-sized stripe-like structure on the NPs in a certain ligand composition.
8
(Figure 2.1 (b) and (c)) This structure is also confirmed by infrared (IR) spectroscopy
9 and more recently by nuclear magnetic resonance (NMR). 10
The stripe-like structure on the NPs forms due to a competition between the enthalpy of phase separation and the conformational entropy that arises at the interface between longer and shorter ligands.
6
When two different ligands are self-assembled, they tend to minimize the amount of interface, and maximize the intermolecular interaction between the same molecules. The enthalpy would be minimized when the same ligand molecules are closely packed. Therefore, enthalpically the stripe NPs are not favored.
However, the conformational entropy of long ligand molecules will increase if they are surrounded by shorter ligands, since there will be additional free volumes around 'extra
25

lengths' of the longer ligands (Figure 2.2 (a)). Therefore, the conformational entropy increases when two different ligands are near by. The balance between enthalpy and entropy creates the phase where the stripe NPs are thermodynamically the most stable as shown by coarse-grained and molecular dynamics simulations.
6
The energy decrease due to the increase of conformational entropy is a significant factor for the stripe structure.
For example, the stripe structure only forms when the size mismatch of two ligands are relatively large. Also, when two ligands with equal length are self-assembled onto the
NPs, the stripe structure forms if one of the ligand has a bulkier end group. However, these simulations
6 as well as experiments" show that the striped NPs only exist in a certain size range.
The smaller NPs phase separate into two bulk phases, so called Janus NPs. This phenomena has been predicted by simulations
6
, and we recently confirmed this in our experiments. 10,
12 Smaller NPs have larger curvatures which provide a larger free-volume to each molecule making the extra-length interfacial conformational entropy mostly irrelevant (Figure 2.2(b)). Therefore, entropy increase is small when two different ligands are closely packed and there is no driving force to overcome the increase in enthalpy when the stripe structure is formed on small NPs. As enthalpy dominates the phaseseparation, the Janus NPs become thermodynamically the most stable structure. In
Chapter 3, we prove that Janus monolayer protected gold NPs only exist in the very small sizes. The synthesis and characterization of Janus NPs are described. The size cut-off for
Janus gold NPs are also determined.
26

(a)
........
e .....
.........
.
.........
II)
Figure 2.2 Schematic drawing illustrating the conformational entropy for ligands on NPs.
On the top, there are cartoons of ligands on NPs, on the bottom there are the predicted NP morphology. For relatively larger radus of curvature, (a) the presence of shorter ligands allows a longer ligand to have a larger free volume; and hence, gain conformational entropy. One can divide the cone that shows a free volume of a long ligand in two parts: the first cone whose angle is determined by the packing of the molecules and the second cone due to this interfacial free volume. (b) As a radius of curvature of a NP decrease, the first cone becomes the dominant factor for the conformation, and the second cone becomes irrelevant.
27

The stripe gold NPs show distinct surface properties from other gold NPs with different surface structures. The surface of the stripe NP coated with hydrophilic and hydrophobic ligands mimics the surfaces that prevent non-specific protein adsorption. it is thought that the alternating regions of hydrophobic and hydrophilic domains are too small for proteins with low configurational energy to adsorb. In addition, they have nonmonotonic solubility 9 and non-monotonic interfacial energy 7 with ligand composition.
Thermodynamically, if two ligands are homogeneously arranged on the surface, a linear relationship between solubility/interfacial energy and the composition of two ligands would be predicted. When we measure the interfacial energy of the striped NPs coated with 1 -octanethiol (OT) and 6-mercapto- 1 -hexanol (MHol) using the contact angle measurement and AFM, the interfacial energy does not increase linearly as the composition of MHol increases. (Figure 2.3(a)) Whereas, when we measure the flat gold surface coated with same two ligands, we found a monotonic behavior of interfacial energy on the average surface composition.
7 Our group also found the similar result for solubility of the stripe gold NPs in various solvents.
9
Figure 2.3 (b) shows the solubility of OT/MPA NPs in methanol and it shows that the maximum solubility of NPs achieved when the composition of MPA is only 33%.
Similar results have been found in molecular recognition , ion-capture 4 and catalysis.
5 Our study also shows that the stripe particles have great cell permeabilityl
6 compared to the gold NPs with non-stripe structures.
The most important property of the stripe NPs related to this thesis is the ability to create divalent gold NPs. The stripe-like structure on the NPs' surface generates two
28

distinct regions at opposite poles where the stripes 'collapse' into point defects. These polar point defects are thermodynamically more reactive and can be chemically modified into divalent NPs that can be reacted to form chains.' 7 In the below, the divalency of the striped gold NPs and the method to create the divalent NPs, so called the 'place-exchange' reaction are described in detail in the next section.
(a)
140S47
CA In wstmr AEM In vatw
SAM on nanoparticles
<100.2 s o
S so
(%MseoI)
100
MM9ol)
I
7SS
!60 X ase
an.
U
0 14 96 so 47 Is
%a of a
1we
Figure 2. 3 Interfacial energy and solubility of the stripe nanoparticles (a) Interfacial energy of OT/MHol NPs in different ligand composition, (b) The solubility of HT/MPA and
OT/MPA NPs in methanol. (reprinted from ref. 7 and ref. 9)
The defect points on the sriped NPs can be explained using topology of vectors on the NP's surface. The Hairy Ball theorem explains the arrangement of vectors on a spherical surface.' 8
,19
According to the theorem, you cannot arrange the vector field on a sphere without forming one or more defect points.
29

Each ligand molecule on a NP can be represented as a vector using its length and tilt angle projected to the surface of a NP. The tilt angles of ligands on a NP are measured based on vectors tangent to continuous spherical surfaces of a NP. They are not measured based on the projected vectors to the faceted surfaces of a NP's actual crystallographic core.
2 0 The ligand arrangement on a NP can be described as an array of vectors onto a spherical NP surface and the possible arrangement of these vectors leads to one or more defect points on the the NP's surface. The defect points of a stripe NP occur on the NP's two poles since there are two vectors from two ligands arrange on a stripe NP's surface.
17
Two ligand molecules on the poles are not surrounded by other ligand molecules with optimum tilt angle, thus they are not least stable. These points can be used to provide divalency on a stripe NP via the place-exachange reaction described in next section.
17
Place-exchange is the reaction by which the composition of a original SAM can be changed by exposing it to excess amount of another kind of ligand molecules.
1
'
2
'
When a SAM is in a solution, a dynamic equilibrium that thiolated molecule reversibly adsorb to the surface and desorb to the solution exists.22.23 Therefore, if we put a SAM into the solution of different ligand molecule, after certain period time (from hours to days), a SAM with the different composition is found because the ligand molecule in the solution adsorbed on the surface and the original ligand molecule on the NPs desorb from the NPs by place-exchange. It is well known that place-exchange reaction occurs 1:1 stoichiometry, that is one molecule is adsorbed on the surface, another molecule is desorbed from the surface.
30

It is known that place-exchange reaction occurs faster at defect sites in the SAM on the flat gold surfaces, 2 5 2 6 because the ligand molecules at defect sites are less stabilized by intermolecular interactions. For gold NPs, it occurs first at edges and vertices of gold NPs and then proceeds to terrace sites either by direct place-exchange or
by migration from the initial exchange sites.
24
Based on these studies, we assumed that two defect points on the poles of the stripe gold NPs are more reactive than other sites on the NPs. Therefore, if we dissolve the stripe NPs in the solution containing a new ligand, the two defect points will be selectively exchanged in the early stage of the reaction. If we use the new ligand with different chemical functionality from the existing two ligands, the divalent gold NPs are created. Our group previously proved the existence of these polar defect sites, and generated the divalent gold NPs.1
7
2.5.
Our group previously reported the synthesis of divalent gold NPs and making chains of NPs.1 The thermodynamic and kinetics studies of the reactivity of the two poles of the striped NPs were also investigated.
2
From previous studies, we demonstrated that the selective functionalization of poles of gold NPs results the divalent gold NPs. When 11 -mercaptoundecanoic acid (MUA) was used for place-exchange, the divalent gold NPs with two carboxylic acid groups was created. (Figure 2.4. Reaction 1)
Then, we used carboxylic acid groups to react with hexamethylene diamine via interfacial polymerization, and found chains of gold NPs. (Figure 2.4. Reaction 2) The existence of chains was verified by TEM and dynamic light scattering (DLS). The kinetic study of
31

pole-functionalization has found that faster place-exchange occurs as the length of the existing ligands on NPs decreases.
2 7 This indicate that if the ligand length is short, the pole would be more exposed, therefore, more poles would be place-exchanged at given time. Therefore, the more chains were formed when there was shorter existing ligand.
In this work, we studied the kinetics of pole-functionalization with different ligand combinations. The brief experimental details and the summary of the work are presented below.
Reaction 1
_
+ X
Reaction 2
M.
0
24 hou~rs
VL
Precttale
+
Unreacted MUA
Figure 2. 4 1-D assembly of the stripe nanoparticles (left) Two step reactions in order to create the divalent gold NPs and the chains using the divalent gold NPs, (right) TEM image of the divalent gold NP chains (scale bar: 50 nm) (reprinted from ref.
2)
Synthesis of 1-nonanethiol (NT)/1-methylbenzylthiol (MBT) NPs with different ligand compositions has been done using modified Stucky synthesis.
28
Five different NPs were synthesized with the stoichiometric ratio of NT/MBT 1:10, 1:2, 1:1, 2:1, 10:1. First,
32

[(C6H5)3P]AuCl (223.06 mg, 0.45 mmol) was dissolved in toluene (20 mL). Then, the mixture of thiols was added to the gold solution and stirred for 20 minutes. While stirring, the temperature is increased to 55 *C. After 20 minutes, when the temperature reached at
55
0
C, tert-butylamine-borane complex (217.43 mg, 2.5 mmol) that dissolved in toluene
(20 mL) was added to the gold-thiol solution. . The solution slowly became yellowish brown then, turned to darker black-brown when the reducing agent was added. The reaction took place for 2 hour at 55
0
C. After the reaction was done, ethanol was added to the solution and the solution was placed in the refrigerator for overnight precipitation.
The precipitation collected using vacuum filter, and washed several times using ethanol, methanol and DI water. Particle size distribution is measured by TEM images, the average core diameter was approximately 4-5 nm, and the difference between batches were negligible.
2.5.2. Pole-Functionalization of Gold Nanoparticles
Pole-functionalization of the NPs was done using the similar method that our group previously has done.
17
2 mg of the NPs was dissolved in THF (66.7 ptL), and different molar excess of MUA (from 5-fold molar excess to 25-fold molar excess), which dissolved in 66.7 pL of THF was added to the NP solution. The solution was stirred for 15 minutes. After the reaction was stopped, the solution was filtered using
Sephadex column. The collected solution was air-dried for the further reaction.
2.5.3. Polymerization of Divalent Gold Nanoparticles
The pole-functionalized NPs were dissolved in toluene, and aqueous solution of hexamethylene diamine was added. Toluene and water create interface, and polymerization between the divalent NPs with MUA and diamine molecules takes place
33

at the interface. The reaction was place for 24 h. After 24h, significant amount of NPs in toluene phase has disappeared, and solid precipitation was formed at the interface.
2.5.4. Results
The fraction precipitated after polymerization has measured by using UV-VIS spectroscopy. It is done two steps: first, UV adsorption of gold NPs in toluene at 520 nm was measured before adding diamine molecules; then, UV adsorption of gold NPs solution at 520 nm was measured after 24 hours when the polymerization was done. The difference in UV adsorption was calculated as fraction precipitated in Figure 2.3.
Assuming that all pole-functionalized NPs participated in polymerization after 24 h, the fraction precipitated indicates the fraction of NPs that pole-functionalized with MUA. As the molar excess of MUA increased in place-exchange reaction, the more polymerization took place, which meant more poles have been exchanged to MUA. No significant change in kinetics of pole-functionalization by varying ligand ratio. (Figure 2.3) It indicates the local environment around the pole would be more important than the overall ligands arrangement on the gold NPs. Even the ligand ratio differs from 10:1 to 1:10, the local environment that creates the poles would be the similar if two ligands are not changed. Therefore, place-exchange reaction rate would be similar regardless of overall ligand ratio.
34

1.0
-0
0.8
(6D cc.
0.6
L..
C
0
0.4
0.2
V
/ A
. .
S
0 0flL
0.0 5.0x104
S
V v v
A
0'~
7
"U
*
S1:2 R2= 0.81
*
S2:1 R2 0.83
-U-
:2 R=
A 0.81
10:1 R 2=
0.98
-,1:10 R =
0.85
.
I
1.5x10
:
3 . 2.0x103 2.5x103
.
Figure 2. 5 The effect of ligand ratio on the polymerization of the striped NPs. The fraction precipitated (amount of pole-functionalized NPs in toluene solution) had no relation with the ligand ratio of the gold NPs.
35

2.5. References
1. Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M., Selfassembled monolayers of thiolates on metals as a form of nanotechnology. Chemical
Reviews 2005, 105 (4), 1103-1169.
2. Stranick, S. J.; Parikh, A. N.; Tao, Y. T.; Allara, D. L.; Weiss, P. S., Phase-
Separation of Mixed-Composition Self-Assembled Monolayers into Nanometer-Scale
Molecular Domains. Journal ofPhysical Chemistry 1994, 98 (31), 7636-7646.
3. Stranick, S. J.; Atre, S. V.; Parikh, A. N.; Wood, M. C.; Allara, D. L.; Winograd,
N.; Weiss, P. S., Nanometer-scale phase separation in mixed composition self-assembled monolayers. Nanotechnology 1996, 7 (4), 438-442.
4. Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M., Self-Assembled Monolayers of
Alkanethiols on Gold Comparisons of Monolayers Containing Mixtures of Short-Chain and Long-Chain Constituents with Ch3 and Ch2oh Terminal Groups. LANGMUIR 1992,
8 (5), 1330-1341.
5. Smith, R. K.; Reed, S. M.; Lewis, P. A.; Monnell, J. D.; Clegg, R. S.; Kelly, K. F.;
Bumm, L. A.; Hutchison, J. E.; Weiss, P. S., Phase separation within a binary selfassembled monolayer on Au{ 111
driven by an amide-containing alkanethiol. Journal of
Physical Chemistry B 2001, 105 (6), 1119-1122.
6. Singh, C.; Ghorai, P. K.; Horsch, M. A.; Jackson, A. M.; Larson, R. G.; Stellacci,
F.; Glotzer, S. C., Entropy-mediated patterning of surfactant-coated nanoparticles and surfaces. Physical Review Letters 2007, 99 (22), -.
7. Kuna, J. J.; Voitchovsky, K.; Singh, C.; Jiang, H.; Mwenifumbo, S.; Ghorai, P. K.;
Stevens, M. M.; Glotzer, S. C.; Stellacci, F., The effect of nanometre-scale structure on interfacial energy. Nature Materials 2009, 8 (10), 837-842.
8. Jackson, A. M.; Myerson, J. W.; Stellacci, F., Spontaneous assembly of subnanometre-ordered domains in the ligand shell of monolayer-protected nanoparticles.
NA TURE MA TERIALS 2004, 3 (5), 330-336.
9. Centrone, A.; Penzo, E.; Sharma, M.; Myerson, J. W.; Jackson, A. M.; Marzari,
N.; Stellacci, F., The role of nanostructure in the wetting behavior of mixed-monolayerprotected metal nanoparticles. Proc. Natl. Acad. Sci. U. S. A. 2008, 105 (29), 9886-9891.
10. Liu, X.; Yu, M.; Kim, H.; Mameli, M.; Stellacci, F., Determination of monolayerprotected gold nanoparticle ligand-shell morphology using NMR. Nature
Communications 2012, 3.
11. Carney, R. P.; DeVries, G. A.; Dubois, C.; Kim, H.; Kim, J. Y.; Singh, C.; Ghorai,
P. K.; Tracy, J. B.; Stiles, R. L.; Murray, R. W.; Glotzer, S. C.; Stellacci, F., Size limitations for the formation of ordered striped nanoparticles. Journal of the American
Chemical Society 2008, 130 (3), 798-799.
12. Kim, H.; Carney, R. P.; Reguera, J.; Ong,
Q.
K.; Liu, X.; Stellacci, F., Synthesis and Characterization of Janus Gold Nanoparticles. Advanced Materials 2012, 24 (28),
3857-3863.
36

13. Liu, X.; Hu, Y.; Stellacci, F., Mixed-Ligand Nanoparticles as Supramolecular
Receptors. Small 2011, 7 (14), 1961-1966.
14. Cho, E. S.; Kim, J.; Tejerina, B.; Hermans, T. M.; Jiang, H.; Nakanishi, H.; Yu,
M.; Patashinski, A. Z.; Glotzer, S. C.; Stellacci, F.; Grzybowski, B. A., Ultrasensitive detection of toxic cations through changes in the tunnelling current across films of striped nanoparticles. Nature Materials 2012, 11 (11), 978-985.
15. Ghosh, A.; Basak, S.; Wunsch, B. H.; Kumar, R.; Stellacci, F., Effect of
Composition on the Catalytic Properties of Mixed-Ligand-Coated Gold Nanoparticles.
Angew. Chem.-Int. Edit. 2011, 50 (34), 7900-7905.
16. Verma, A.; Uzun, 0.; Hu, Y. H.; Hu, Y.; Han, H. S.; Watson, N.; Chen, S. L.;
Irvine, D. J.; Stellacci, F., Surface-structure-regulated cell-membrane penetration by monolayer-protected nanoparticles. NA TURE MA TERIALS 2008, 7 (7), 5 88-595.
17. DeVries, G. A.; Brunnbauer, M.; Hu, Y.; Jackson, A. M.; Long, B.; Neltner, B. T.;
Uzun, 0.; Wunsch, B. H.; Stellacci, F., Divalent metal nanoparticles. Science 2007, 315
(5810), 358-361.
18. Nelson, D. R., Toward a tetravalent chemistry of colloids. Nano Letters 2002, 2
(10), 1125-1129.
19. Eisenberg, M.; Guy, R., Proof of the Hairy Ball Theorem. American
Mathematical Monthly 1979, 86 (7), 572-574.
20. Luedtke, W. D.; Landman, U., Structure and thermodynamics of self-assembled monolayers on gold nanocrystallites. Journal ofPhysical Chemistry B 1998, 102 (34),
6566-6572.
21. Ulman, A., Formation and structure of self-assembled monolayers. Chemical
Reviews 1996, 96 (4), 1533-1554.
22. Biebuyck, H. A.; Bian, C. D.; Whitesides, G. M., Comparison of Organic
Monolayers on Polycrystalline Gold Spontaneously Assembled from Solutions
Containing Dialkyl Disulfides or Alkenethiols. LANGMUIR 1994, 10 (6), 1825-1831.
23. Biebuyck, H. A.; Whitesides, G. M., Interchange between Monolayers on Gold
Formed from Unsymmetrical Disulfides and Solutions of Thiols Evidence for Sulfur
Sulfur Bond-Cleavage by Gold Metal. LANGMUIR 1993, 9 (7), 1766-1770.
24. Hostetler, M. J.; Templeton, A. C.; Murray, R. W., Dynamics of place-exchange reactions on monolayer-protected gold cluster molecules. LANGMUIR 1999, 15 (11),
3782-3789.
25. Collard, D. M.; Fox, M. A., Use of Electroactive Thiols to Study the Formation and Exchange of Alkanethiol Monolayers on Gold. LANGMUIR 1991, 7 (6), 1192-1197.
26. Lee, L. Y. S.; Lennox, R. B., Electrochemical desorption of n-alkylthiol SAMs on polycrystalline gold: Studies using a ferrocenylalkylthiol probe. LANGMUIR 2007, 23 (1),
292-296.
37

27. DeVries, G. A.; Talley, F. R.; Carney, R. P.; Stellacci, F., Thermodynamic Study of the Reactivity of the Two Topological Point Defects Present in Mixed Self-Assembled
Monolayers on Gold Nanoparticles. Advanced Materials 2008, 20 (22), 4243-4247.
28. Zheng, N.; Fan, J.; Stucky, G. D., One-step one-phase synthesis of monodisperse noble-metallic nanoparticles and their colloidal crystals. JOURNAL OF THE AMERICAN
CHEMICAL SOCIETY 2006, 128 (20), 65 50-65 5 1.
38

When two dislike molecules self-assemble on the gold nanoparticle (NP) surface, the phase separation occurs as described in chapter 2. It has been predicted theoretically' that when the size of the gold core becomes smaller, the interfacial entropy contribution becomes less and less relevant, since the increase in the cone angle that delimits the ligands yields an overall increase in conformational entropy for the entire chain (Figure
2.2). Under this condition, the final morphology is primarily determined by the enthalpy of separation; hence, for small particles, two ligands phase separate into two bulk phases, resulting the Janus NPs. In this chapter, we demonstrate that gold NPs with a core diameter smaller than 1.5 nm form Janus NPs in many ligand combinations. We synthesize four different types of gold NPs with varying ligand combinations and characterize them using 2D NMR, analytical ultracentrifugation (AUC), gel electrophoresis (GEP), and transmission electron microscopy (TEM). All of these techniques confirm the presence of a majority of Janus particles and show similar cut-off sizes for the Janus-to-stripe transition.
Janus particles are one of the most studied types of patchy particles, due to its relatively simple patchy structure. In past decade, many efforts have been done to make
Janus particles. They have been studied not only for industrial application such as e-ink displays, 2 catalysis,
3 anti-reflective coatings,
4 and emulsifying agents,
5 but also for
39

fundamental studies that establish their assembly properties as well as their use as surfactant particles.-
1 0
Janus particles were studied with different core materials such as polymers (e.g. polystyrene beads, block/triblock copolymer), silica particles, and inorganic particles. In addition, the wide ranges of particle architectures from a sphere to a rod or a disc to a dumbbell were investigated. (Figure 3.x) The compartmentalization is obtained by having two different core materials or by having two different surface modifiers (e.g. thiols, surfactants, and polymers etc.).
Several synthetic methods for the fabrication of Janus particles currently exist.
Examples include microfluidics," biphasic electrohydrodynamic jetting,1 2 layer-by-layer assembly,' 3 self-assembly14-16 and stepwise functionalization of immobilized particles on the template or at the interface.
17-23
Among the various types of Janus particles, only spherical inorganic NPs would be considered in this chapter. Recent progresses in the synthesis of spherical inorganic
NPs are reviewed in detail below, and briefly describe our new approach to make Janus
NPs. The properties of these Janus NPs would be also discussed.
3.1.1. Synthesis of Inorganic Janus Nanoparticles using Solid Template
First, Shumaker-Parry and co-workers synthesized two different Janus NPs using amine-terminated glass slides and coupled them to dimers.
2 3 (Figure 3. 1) The citratestabilized gold NPs of 41 nm is immobilized on amine-terminated glass slides and immersed them in ethanol solution containing 11 -mercapto- 1 -undecanol (MUOH) to replace the upper half of the NPs. Then, the MUOH functionalized NPs were sonicated in ethanol containing either 16-mercaptoundecanoic acid or mercaptoethyl amine. The acid
40

terminated Janus NPs and the amine terminated Janus NPs were reacted into dimers.
They also synthesized the Janus NPs with different sizes, and made hetero-dimers as well.
This group used the similarly developed Janus NPs to react with the side groups of linear polymers and achieved 1 -D assembly of Janus NPs. This is confirmed by TEM, and a red-shift in the Plasmon resonance.
(a)
(T T r T
(b)
A i
-""o V00
of
~
Figure 3. 1 Solid template based synthesis of Janus NPs using amine terminated glass slides (a)
Synthesis of two different Janus NPs using amine functionalized glass slides. (b) (c) The formation of dimers of two different Janus NPs (reprinted from ref.
23 and
24)
Another approach is to use a polymer single crystal for the template. Li and coworkers used thiol-functionalized polyethylene oxide (HS-PEO) single crystals in order to immobilize gold NPs in solution.
1 8 After the NPs are immobilized, the solvent-exposed ammonium groups are exchanged to thiol functionalized ATRP initiator. Then, poly
(methyl methacrylate) (PMMA) or poly (tert-butyl acrylate) (PtBA) was synthesized from the gold surface, and Janus NPs were obtained by dissolving the single crystal substrates. The confirmation of the presence of the both PEO and PMMA or PtBA was confirmed by GPC after iodine decomposition of gold core. After hydrolyze acrylate
41

group of PtBA, PEO/PAA covered gold NPs were obtained, and platinum (Pt) NPs were grown in situ of PAA region. TEM image of Pt-decorated gold NPs have taken to prove that the initial gold NPs has biphasic structure of PEO/PtBA on the surface. They showed that PEO/PMMA Janus NPs self-assemble into worm-like clusters in the mildly selective solvent (dioxane). The control sample of PEO/PMMA mixed ligand covered NPs without
Janus structure, did not form such a cluster which confirmed by a shift of the plasmon peak. This method can be applied to other single crystalline polymers, and wide range of polymers can be synthesized from the surface using ATRP. In addition, this method can create the larger amount of NPs compared to hard template-based methods, since it is solution-based.
SHSH
PEO single C n
MA/PtBA PMMA/PtBA f
PE'
Figure 3. 2 Solid template based synthesis of Janus NPs using polymer single crystals (top) Synthesis of Janus NPs using single crystal of HS-PEO. (bottom) Pt NPs grown in PAA region confirming the formation of Janus structure using this template method (reprinted from ref.
18)
42

Hatton and co-workers used silica beads as a temporary support in order to synthesize Fe
3
0
4
Janus NPs.'2 They used PAA coated Fe
3
0
4
NPs to be attracted on the cationically charged silica beads. After Fe
3
0
4
NPs is immobilized on the silica beads,
PAA is reacted with various amine-terminated polymers, and Janus magnetic NPs were created. Many amine-terminated polymers were able to attach on the PAA coated Fe
3
04
NPs and result Janus structure. They used PSSNa (pH-responsive) and PNIPAAm
(temperature-responsive) for amine-terminated polymers to create stimuli-responsive magnetic NPs that the magnetic Janus NPs associate/dissociate by applied external stimuli.
25
(a)
W
TSP~AC~+
+ +
PAA-Coae NP
2
Naoatk ie
~
'3
(b) pH-Responsive
Janus Nanaparticies
PSSNa
Stable Nanoperticl.
PM- Coated
Nanaparicto
Low pH
High pH
Reversible Stimtuu
TempeaueRespnsive
Janu PAMprtc
NeON to pHt 12
SIOj
Seff-Asemblod
GCuter
Figure 3. 3 Solid template based synthesis of Janus NPs using particle surfaces (a) Synthesis of magnetic Janus NPs using negatively charged Si0
2 particles. (b) Schematic representation of stimulireponsive (pH and temperature) magnetic Janus NPs synthesized decribed in (a) (reprinted from ref.
21 and
25)
43

3.1.2. Synthesis of Inorganic Janus Nanoparticles using liquid-air or liquid-liquid interface
The ligand exchange reaction is the powerful tool in creating Janus NPs at interfaces. Chen and co-workers placed 1-hexanethiol (HT) coated gold NPs at air/water interface of a Langmuir trough and made compressed monolayer.1
9
Then, they added 3- mercaptopropane- 1,2-diol (MPD) in water to exchange HT to MPD. They confirmed the
Janus structure by measuring contact angle of films by Langmuir-Blodgett method. The film made by up-stoke deposition showed the similar contact angle to that of HT coated
NPs films, whereas the film made by down-stroke deposition showed the similar contact angle to that of MPD coated NPs films. The control NPs that is place exchanged in bulk, showed the contact angle between two values of all-MPD coated NPs film and all-HT coated NPs film. In further investigation, the same group has investigated the biphasic structure by 2D NOESY NMR. 2
6
They showed that the cross-peaks present in bulkexchanged NPs disappear for the Janus NPs prepared at the interface.
Recently, Bishop and co-workers has synthesized hydrophobic/hydrophilic Janus
NPs by using liquid/liquid interface.
2 2 They used the place exchange reaction at toluene/water interface. First, 1 -dodecanethiol (DDT) coated gold NPs were dissolved in toluene phase of toluene/water mixture. Then, 1 -mercaptoundecanoic acid (MUA) was added to water phase and after vigorous shaking for 30 min, the Janus NPs of DDT/MUA is formed on the interface. The opposite reaction, which starts from the MUA coated gold
NPs to react with DDT in toluene results the same Janus NPs. The Janus structure is characterized by the contact angle measurement of monolayer of NPs on the hydrophilic
44

and the hydrophobic surfaces. This method can be generally applied to other NPs, and the authors showed the amphiphilic magnetic NPs using iron oxide.
(a)(b
AuDDT
Sinjection intcrfacial cxchange
0
HS -
~
Toluene+
Water
AuMUA
0DDT
AuDDT/MUA
MUA
AuDDT/MUA
Figure 3. 4 Interface based synthesis of Janus NPs (a) The synthesis of Janus gold NPs in air-water interface using Langmuir trough, (b) The synthesis of hydrophobic/hydrophilic Janus NPs using toluene-water interface. (reprinted from ref. 19 and
22)
3.1.3. Synthesis of Inorganic Janus Nanoparticles using self-assembly of ligands
The spontaneous phase separation of ligand molecules on the NPs can provide thermodynamically stable Janus NPs in most efficient way. This method also may be the most scalable method, and the wide variety of ligand combinations may be chosen. Many attempts have been done to achieve the Janus NPs using the self-assembly of two ligands, but each method developed so far has some limitations.
Schenning and co-workers have synthesized gold NPs covered with two different ligands with disulfides, one is hydrophobic ligand, and the other is hydrophilic ligands.'
4
,
45

15
They showed that in certain ligand combination, the NPs self-assemble into disc form in water. The thickness of the disc is verified ~8-8.5 nm, which corresponded to the bilayer of gold NPs. However, the phase separation of two ligands seems incomplete, based on the contact angle measurement of the gold NPs films.
The synthesis of gold NPs with different binding groups has been used to create
Janus gold NPs.1
6 Vilain et. al. synthesized gold NPs coated with diphenylphosphinine, and then, performed place-exchange reaction with the various thiol ligands such as mercaptoundecanoic acid, dodecane thiol, and mercaptobenzene. In order to verify the biphasic structure, UV adsorption is measured before (all-diphenylphosphinine covered
NPs) and after the place exchange reaction (combination of a thiol and diphenylphosphinine). Since two ligands have different electronic interactions with gold core, the electron repartition occurs on the NPs shell when two ligands are completely segregated. This leads to the blue-shift of UV adsorption. They showed the blue-shift of
UV adsorption, after ligand exchange, which indicate thiol and diphenylphosphinine are phase separated. Also, they investigated DDT exchanged NPs using FT-IR, and found homogeneous DDT domain. Finally, MUA exchanged NPs showed different selfassembly behavior depending on the water content in THF/water mixtures. Bilayer structure was found when the ratio between THF/water is 1/1 indicating the biphasic structure of ligands on the NPs. This method is promising method to create Janus NPs, but the possible ligand combination may be limited.
Our group has synthesized the gold NPs with various combinations of ligands, and characterized their phase-separated structure on the gold surface as described in
Chapter 2. The Janus structure has been predicted in very small NPsI, but it has not been
46

experimentally verified. In this chapter, we introduce a new method to synthesize Janus monolayer-protected gold NPs using self-assembly of two ligands. All previous methods described above involve multiple steps in order to obtain controlled Janus structure. The synthetic method that we developed is one-step method, which is easily scalable with high yield of Janus NPs.
3.2. Synthesis of Janus Monolayer Protected Gold Nanoparticles
Many works has been done to control the size of gold nanoparticles. In general, the size of NPs can be changed by varying reaction temperature, the mixing time between gold salt and ligands, gold salt-to-ligands ratio, solvent combination, and ligand-toreducing agent ratio.
Gold NPs were synthesized using four different ligand combinations in order to demonstrate that the Janus morphology is universally present in small monolayer protected gold NPs. NPs with small core diameters were synthesized using the one-phase method 2 7 and a modified Stucky method.
2 8
The synthesis of the Janus NPs described in this chapter has been done by either conducting the reaction at low temperature (0 C), keeping the short mixing time (- 10 min) between the gold salt and two ligands before adding the reducing agent, or increasing the composition of the bad solvent in the solvent mixture. We also tried to synthesize the broadly distributed gold NPs to determine the limit of Janus gold NPs.
Following four gold NPs are synthesized considering four different characterizations:
47

1) NPs coated with 1:1 ratio of hexadecane- 1 -thiol (HDT) and 1,1',4', 1"- terphenyl-4-thiol (TPT)
2) NPs coated with 2:1 ratio of tetradecane-1-thiol (TDT) and 1 1-amino-Iundecanethiol (AUDT)
3) NPs coated with 1:1 ratio of 3-mercapto-1-propanesulfonate (MPSA) and
N,N,N-trimethyl(I 1 -mercaptoundecyl) ammonium bromide (TMA)
4) NPs coated with 1:1 ratio of 1-octanethiol and 3-mercapto-1-propanesulfonate
(MPSA)
The ligand composition used in this chapter refers the stoichiometric ratio used in the reaction. We know the real ligand composition on the NPs' surface differs from the initial reaction condition
29
, but the difference is not relevant for the argument in this chapter. The general synthesis method and the condition varied to change the size of NPs are described in this section. The detailed synthesis procedures are described in section
3.4.
3.2.1. Synthesis of HDT/TPT covered gold nanoparticles
Gold NPs covered with HDT/TPT ligands were synthesized using a modified
Stucky synthesis.
2
8
The ligand combination is selected to perform nuclear Overhauser enhancement spectroscopy (NOESY). Two batches of NPs with different sizes were synthesized; one batch has the small average core diameter of
-
2 nm, and the other batch has the large average core diameter of- 4.5 nm. In order to control the size of NPs, the small NPs are synthesized at 0 'C, and the large NPs are synthesized at 20 'C. All other conditions are maintained same for the both syntheses.
48

3.2.2. Synthesis of TDT/AUDT covered gold nanoparticles
Gold NPs covered with TDT/AUDT ligands were synthesized via a modified
Stucky synthesis.
2
8 In this case we used the mixture of solvent to control the size of the gold NPs. In general, when we use the mixture of the good solvent and the bad solvent of
NPs, the size of NPs gets smaller when the portion of bad solvent is increased. For this synthesis, the mixture of solvents 1:2 of chloroform:methanol was used. This ligand combination is chosen to characterize its Janus structure by chemical linking using amine end-group of AUDT.
3.2.3. Synthesis of MPSA/TMA covered gold nanoparticles
Gold NPs covered with MPSA/TMA ligands were synthesized using the onephase method. The ethanol was used for the solvent, and very small amount of methanol is used to completely dissolve the both ligands before adding NaBH
4
. In order to obtain the small NPs, the synthesis is performed at 0 'C. MPSA and TMA are oppositely charged ligands, so that if they phase-separate into the Janus morphology on the gold NPs, the each NP would act as a large dipole. Therefore, we hypothesized that it is possible to separate between Janus NPs and non-Janus NPs under electric field. We selected this ligand combination for further characterization using gel electrophoresis
(GEP).
3.2.4. Synthesis of OT/MPSA covered gold nanoparticles
Gold NPs covered with OT/MPSA ligands were synthesized using the one-phase method.
2 7 The ethanol was used for the solvent, and very small amount of methanol is used to completely dissolve the both ligands before adding NaBH
4
. In order to obtain the small NPs, the synthesis is performed at 0 'C, and the mixing between gold salt and the
49

ligands mixture kept short (10 minutes). The negatively charged MPSA molecule is used for self-assembly of Janus NPs when divalent cation is introduced.
Many Janus NPs that described in section 3.1 were determined to be Janus using the contact angle measurement. However, this approach was not applicable to our method because our Janus NPs were too small to be effectively immobilized onto a surface. In addition, since the size distribution of our NPs was broad, the single batch is mixture of
Janus NPs and other patchy NPs which requires the separation of Janus NPs in order to conduct the contact angle measurement. Therefore, we used different methods to verify the ligand shell structure of our NPs, tried to estimate cut-off limit, and separated Janus
NPs from the striped/patchy NPs.
3.3.1. Nuclear Overhauser Enhancement Spectroscopy (NOESY) of
HDT/TPT gold nanoparticles
First, we investigated the morphology of HDT and TPT coated NPs using nuclear
Overhauser enhancement spectroscopy (NOESY). A recent study by Pradhan et al.
showed that NOESY can be used to determine Janus morphology on NPs' ligand shell, because when biphasic separation of ligands occurs, cross-peaks between the hydrogen on one type of ligand molecules and the hydrogen on the other type nearly disappear.
2
6
Recent work in our group by Liu et al. has confirmed this approach as well.
2 9
In order to use NOESY, we chose HDT and TPT, since their 'H chemical shifts do not overlap with each other. Since NOESY shows cross-peaks only for short-range dipole-dipole
50

interactions between nuclear spins, the cross-peaks between HDT and TPT would exist only when two molecules are close together.
In the case of Janus NPs, there should be a very weak cross-peak or no cross-peak because only a relatively small interface between two ligands exists. To perform our study, we chose a comparative approach synthesizing particles of two different sizes (the core diameters of -2.0 nm and -4.5 nm) with identical ligand shell composition. The
NOESY spectra of these two NPs covered with HDT/TPT are shown in Fig. 3.5. The
NOESY NMR spectrum of small gold NPs (-2.0 nm) shows no cross-peaks between
HDT and TPT (Fig. 3.5(a)). In clear contrast, obvious cross-peaks are observed with the larger size (- 4.5 nm) NPs (Fig. 3.5(b)). This result confirms that gold NPs with small cores form Janus NPs.
(b) r'j
104
10
8 6 4 2 0 F2 [ppmI 10 8 6 4 2 0F2 [ppm]
Figure 3.5 NOESY measurements of HDT/TPT mixed ligand coated nanoparticles.
(a) NOESY measurement of Janus nanoparticles; (b) NOESY measurement of striped nanoparticles with the same ligand composition. Clear cross-peaks shown in striped nanoparticles (red box) disappear in small nanoparticles, which indicates two ligands phase-separate as Janus nanoparticles.
51

Janus structure can also be confirmed by chemically linking gold NPs. The similar approach has been done by Sadar et.al.i To achieve this goal, NPs coated with a
2:1 ratio of TDT/AUDT were synthesized to make the amine group at the end of AUDT react with sebacoyl chloride (SC), leading to dimers of NPs. Our hypothesis is that Janus particles will form mostly dimers in the right reaction conditions, while dimers would be an unlike reaction product for the striped or other patchy NPs. We used the ligand ratio of
TDT/AUDT 2:1 so that the surface covered with amine-group is relatively small, rather than a half hemisphere of NPs. This would help the reaction to form mostly dimers because once a dimer is formed, there would be significant steric hindrance preventing the third particle from reacting with the small area of AUDT on the surface.
TDT/AUDT NPs were reacted with ~10-fold molar excess of SC in dichloromethane for 10 min, and were filtered using Sephadex column. NPs were separated into two populations during filtration. One population moved through the column, and the other population remained on the top of the column and did not go through even after adding the sufficient amount of dichloromethane is added. The population did not go through the column is separated from the column and dissolved back in dichloromethane for TEM. Both fractions were characterized using TEM (Fig.
3.6). Dimers could be found only in the fraction that did not go through the column, indicating that these aggregates were covalently bonded during the reactions and were not a dynamically self-assembled product. We analyzed TEM images to determine the size of particles that formed dimers. As shown in Fig. 3.6 (c), only NPs smaller than 3 nm formed dimers. Also, the average diameter of NPs forming dimers was 1.8 ± 0.4 nm. In
52

addition to TEM analysis, we performed analytical ultracentrifugation (AUC), a technique allowing the detection of dimers in solution. AUC measures sedimentation properties of materials that depend on size, shape, and density of the material. It is widely used in biology to separate and characterize proteins in their mass and structure. Recently, our group used AUC to determine the size, density, and molecular weight of gold NPs dispersed in solution.
3 0 AUC experiments were performed before and after dimerization, and the change in size and shape distribution of NPs was measured. AUC data were first used to confirm the size of the particles determined by TEM (Fig. 3.6 (d)). We then used
AUC to analyze the effect of the dimerization reaction. As evident from the comparison of figure 3.6 (c) and 3.6 (d), a sizeable part of the fraction of particles in the sample disappeared from the AUC scan after dimerization, while at the same time a new peak
(that we assigned to the dimers) appeared. The small particles had a sedimentation coefficient of 35 S while the dimers had a sedimentation coefficient of 90 S. These values were converted to hydrodynamic diameter using a modified Svedberg equation and are shown at the top of Fig. 3.6 (d). The AUC results confirm that the dimers exist in the solution, and they are formed only from the small sized NPs. The both TEM and AUC results confirms that NPs that are smaller than 3 nm can be Janus NPs.
53

(a)
2 4 -S-fCH
(b)
CI)CI
(c)
80
I
0
C
0
C.)
40 -
.
-N--CH s-N-
(d)
0
I
0
3
1 2 3
. . . . .
. .
4 5
Core Diameter/nm
Hydrodynamic Diameter/nm
4 S
6
I
.2
/ 3
0 50 100
Sedimentation Coefficient /S, 10 s
150
Figure 3. 6 (a) Dimerization scheme of TDT/AUDT Janus nanoparticles; (b) TEM image of NPs fractions that went through the Sephadex column (top) and that remained on the top of the column
(bottom). Dimers are shown in yellow box. Scale bar: 20 nm; (c) the size distribution of the particles in dimers (red) and in non-dimers (blue), indicates that only smaller sizes of nanoparticles participate in dimerization; (d) AUC result of Janus nanoparticles before (left) and after (right) dimerization: strong peak at 35s disappears after dimerization and new peak appears at 90s, which shows dimers of 35s have formed.
54

3.3.3. Gel electrophoresis (GEP) of MPSA/TMA gold nanoparticles
We synthesized gold NPs covered with oppositely charged ligands, MPSA and
TMA. We reasoned that Janus NPs would show an electrostatic behavior that would be very different from the electrostatic behavior of the striped NPs or that of any other NPs with more homogenous distribution of ligand molecules. Janus particles would act as large dipoles, while all other distributions would have a multi-polar or more generically zwitterionic behavior. A recent theoretical paper also shows agreement with our reasoning.
Gel electrophoresis (GEP) is widely used in biology to separate DNA and other macromolecules, using differences in size and charge density/distribution. Thus, we argued that Janus particles could be separated from other particles due to the differences in electrostatic spatial distribution on their surface. A broadly size-distributed NP batch was synthesized to separate the NPs using GEP. TEM showed that the NPs had a
Gaussian size distribution without hint of a bi- or multimodal distribution. However, we observed that the NPs separate into at least two distinct fractions when GEP is performed: one fraction traveled with the potential, and another mostly remained still and had a long tail that moved a lot less than the initial population. (Fig. 3.7) In stark contrast, homoligand NPs coated with MPSA or TMA traveled as a single population. (Fig. 3.7 (a))
We interpret the clear separation into two populations of the MPSA/TMA NPs as the separation between Janus and non-Janus NPs.
To test this hypothesis, we extracted the two populations from the gel and characterized them by TEM to determine the size of the NPs. As shown in Fig. 3.7 (b), smaller NPs are present in the first population (pop 1), while larger NPs are present in the
55

second (pop2) population. It should be stressed that there is no gap in the size distribution of these two populations. The interpretation of our results is that pop 1 is made of Janus
NPs that traveled faster than striped NPs because of their dipolar nature, but they traveled slightly less than similarly sized (average size of 1.6 nm) all-MPSA NPs that bear a larger negative charge. Pop2 contained larger non-Janus particles where both ligands roughly balance opposite charges; hence, they showed the little mobility in the gel. The average size of NPs that moved furthest (pop1) is 1.5±0.5 nm, while that of NPs that moved less (pop2) is 3.8±2.4 nm. Dimers and long chain-like assemblies of Janus NPs described in the literature
7,8 are found in the population of the smallest NPs, which also indicates that the small NPs are Janus NPs. TEM of the popi and pop2 is shown in Figure
3.8.
(a) (b)
POP2
_A/TMA NPs 'E
30
M+ pop, pop2 H-
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Diameter (nm)
Figure 3.7 (a) Gel electrophoresis of TMA/MPSA Janus NPs; (b) the size distribution of NPs separated into popl (red), and pop2 (blue) after gel electrophoresis.
56

20 no
Figure 3.8 TEM image of separated MPSA/TMA NPs (top) the population that did not move under electric field after GEP, (bottom) the population that moved furthest (popl in
Figure 3.7) Linear assemblies of Janus NPs were shown in red box.
Finally, we synthesized gold NPs covered with OT and MPSA. The amphiphilic
OT/MPSA NPs were assembled by adding divalent cations in the solution. We used Ca 2
+ ion to bring MPSA ligands close together (Figure 3.9 (a)). TEM image has taken for NPs before and after adding Ca
2
+ ions. Dimers were found after adding cations, and the separation between small and large NPs were visible in the image. Only the smaller size of nanoparticles participated in dimerization (Figure 3.9 (b)), and the other possible
57

assemblies of typical Janus nanoparticles were also found under TEM (Figure 3.9 (c)).
(a) Ca2+/ water
* and other possible assemblies
(b) (c)
0,0 0 5
1'0 is 2,0 2
5
3
0
Particle Diameter (nm) e
Figure 3. 9 (a) Dimerization scheme of OT/MIPSA Janus nanoparticles, (b) The size distribution of the particle participate in dimer (blue) and in non-dimer (green), (c) TEM image of the assembly, assembled particles are shown in color Scale bar: 20 nm
The amphiphilic Janus NPs were tested its ability to stabilize the emulsion at toluene/water interface. The emulsion stabilized by solid colloidal particles is specially named as Pickering emulsion. 3 It is known that amphiphilic Janus nanoparticle lowers oil-water interfacial tension compared to uniform metallicnanoparticles of the similar size.5 We used two different sizes of OT/MPSA NPs. The average size of 1.9 nm and 3.5 nm represents Janus NPs and non-Janus NPs respectively. OT/M4PSA NPs were dissolved in water, and small amount of toluene was added on top of water. After vigorous agitation for a few minutes, the emulsions containing toluene are formed in water.
However, the stable Pickering emulsion was formed only in the solution containing Janus
NPs, and the solution with non-Janus NPs immediately separated back to toluene and water/NPs phases. The emulsions formed by Janus NPs remain stable for several days.
58

Emulsion polymerization using pickering emulsion of polymeric Janus nanoparticles
33
, and homoligand capped magnetic nanoparticles 3 4 has been reported. Using the same idea, polystyrene (PS) was synthesized in the emulsion using Free Radical Polymerization.
TEM image clearly shows the PS in the emulsion. (Figure 3.10 (b)) water
+
water
Figure 3. 10 (left) Pickering emulsion of Janus nanoparticles in toluene/water interface, scale bar: 20 nm. (right) Polystyrene synthesized via Free Radical Polymerization in the Pickering emulsion in toluene/water interface, scale: 100 nm.
Based on dimerization and GEP, we tried to determine the size limit for Janus
NPs. Dimerization data (as shown in Figure 3.6 (c)), indicate that Janus NPs exist when the core diameter is 1.5 nm or smaller and are not found for NPs larger than 3 nm. It is likely that NPs with sizes between 1.5 and 3.0 nm have either Janus or patchy morphology. These results agree well with our previous studies showing that only NPs larger than 2.0 nm have striped morphology.
3 5
A similar trend was observed when analyzing the results of the GEP experiments and dimers formed by ionic interaction.
59

Therefore, we can conclude that Janus NPs exist when the core diameter is smaller than
1.5 nm, and the transition between Janus-to-striped NPs observed between 1.5-3.0 nm.
However, it should be noted that this range is not absolute and it will depend critically on the nature of the ligand mixture used. For example, in our STM analysis of
MUA/OT NPs, we see Janus NPs for NPs of -5.8 nm in size.
36 It is always difficult to determine the core size of NPs precisely by using STM images, 37 but if we use the estimation of subtracting twice the extended ligand length, we obtain that the core size of these Janus NPs is 3.1-3.2 nm. This indicates that these NPs exist in the Janus morphology at slightly larger sizes than NPs synthesized in this chapter. The origin of the change in the size limit can be explained by the length mismatch between MUA and OT ligands. The length mismatch between MUA and OT is the smaller compared with the ligand combinations that we characterized in this chapter. It agrees with our understanding that the Janus-to-striped transition will occur at larger sizes when the length mismatch between two ligands becomes smaller.'
In summary, we synthesized Janus gold NPs in various ligand combinations. The synthetic methods used here directly developed Janus structure without any templates or further modification, and the majority of NPs were Janus NPs; hence, they are available in high-yield. We also determined that gold NPs covered with two immiscible ligands form stable Janus NPs when the metal cores are smaller than 1.5 nm. Based on the size analysis, NPs larger than 3 nm would not form Janus NPs. The mixed morphologies of
60

Janus and striped NPs are predicted to exist when the gold core size is between 1.5 3 nm.
Tetradecane- 1 -thiol (TDT), hexadecane- 1 -thiol (HDT), 1 -octanethiol (OT),
1,1 ',4',1 "-terphenyl-4-thiol (TPT), 3-mercapto- 1 -propanesulfonate (MPSA), N,N,N- trimethyl( 11 -mercaptoundecyl)ammonium bromide (TMA), 11 -mercaptoundecanoic acid
(MUA), gold(III)chloride trihydrate (HAuCl4 3H20), chloro(triphenylphosphine) gold(I)
([(C6H5)3P]AuCl), sodium borohydride (NaBH4), tert-butylamine-borane complex morpholine borane complex, agarose, and lipophilic Sephadex were purchased from
Sigma-Aldrich, and used as received. 11-Amino-i -undecanethiol hydrochloride (AUDT) was purchased from ProChimia Surfaces, and used as received. All solvents were used as received. MilliQ water (EMD Millipore) was used when water is needed. Carbon coated
TEM grids were purchased from EMS Acquisition Corp.
3.6.2. Synthesis of gold nanoparticles
Synthesis ofHDT/TPT gold NPs: [(C6H5)3P]AuCl (123.68 mg, 0.25 mmol) was dissolved in dichloromethane (20 mL). The 1:1 mixture of thiols was added (HDT, 16.2 mg; TPT, 16.4 mg; 0.1225 mmols in total) and stirred for 10 minutes. tert-butylamineborane complex (217.43 mg, 2.5 mmol) was dissolved in dicholoromethane (20 mL), and it was added to gold-thiol mixture. The clear solution became brown and then turned darker. After addition of all of the reducing agent, the solution was stirred for 2 hours.
When the reaction is finished, ethanol was added and the solution was placed in a
61

refrigerator for overnight precipitation. The precipitation was filtered either using centrifuge, or vacuum filtration with quantitative filter paper and it was washed with ethanol, methanol, acetone and water. Particle size distributions were measured by TEM.
Synthesis of TDT/A UDT gold NPs: [(C6H5)3P]AuCl (123.68 mg, 0.25 mmol) was dissolved in the solvent mixture (10 mL). The 2:1 mixture of TDT:AUDT (TDT, 90.8 [iL;
AUDT, 39.75 mg; 0.5 mmol in total) was dissolved in the solvent mixture (10 mL in total) and then mixed with the gold solution and stirred for 10 min. tert-butylamine-borane complex (217.4 mg, 2.5 mmol) in the solvent mixture (20 mL) was added to the goldthiol mixture. The solution was immediately put at 60 'C and left to react during one hour.
Then the reaction was left to cool for 30 min. Part of the solvent was removed via rotary vacuum evaporation, and dimethylformamide was added to induce NPs precipitation. The solution was left to precipitate for overnight and the supernatant was subsequently removed. The rest of the purification was done with several cycles of centrifugation in acetone. Particle size distributions were measured by TEM.
Synthesis ofMPSA/TMA gold NPs: Gold NPs covered with MPSA/TMA ligands were synthesized using one-phase method in ethanol.
2 7 Before adding chemicals, the solvent (ethanol) was purged in nitrogen for 30 min, and all the steps thereafter were performed in a nitrogen environment. 0.45 mmol of HAuCl
4
-3H
2
0 (177.2 mg) were dissolved in 100 mL of ethanol at 0 'C. 0.45 mmol of 1:1 thiol mixtures (MPSA, 40.1 mg;
TMA, 63.4 mg) were added to gold solution and stirred for 10 minutes. Then, 10 mmol of
NaBH
4
(189.1 mg) dissolved in 100 mL of ethanol were slowly added drop-wise using a syringe needle. Upon addition, the gold-thiol solution became turbid yellowish-green and slowly darkened to purplish-black. After the complete addition of NaBH
4 solution, the
62

solution was stirred for 3 hours. Then it is placed in the refrigerator for overnight precipitation. The precipitate was filtered using vacuum filtration with quantitative filter paper, and washed with ethanol, methanol, and acetone, obtaining a shiny dark powder.
Synthesis ofMPSA/OT gold NPs: Gold NPs covered with the MPSA/OT ligands were synthesized in the same method that we described for MPSA/TMA NPs. This synthesis is done under nitrogen environment. 0.45 mmol of HAuCl4-3H
2
0 (177.2 mg) were dissolved in 100 mL of ethanol at 0 'C. 0.45 mmol of 1:1 thiol mixtures were added and stirred for 10 minutes. Then, 10 mmol of NaBH
4 dissolved in 100 mL of ethanol was slowly added dropwise using syringe needle. After the complete addition of NaBH
4 solution, the solution was stirred for 3 hours. Then it is placed in the refrigerator for overnight to get precipitation. The precipitation was filtered using vacuum filtration with quantitative filter paper, and washed with ethanol, methanol, acetone and very small amount of water.
3.6.3. NOESY
NOESY NMR experiment was done by X. Liu, and the details of this experiment is explained in reference 29
3.6.4. Dimerization of TDT/AUDT NPs
Chemical dimerization method was used for amine terminated gold NPs
(TDT/AUDT NPs). We used sebacoyl chloride (SC) as the chemical linker of two particles. TDT/ AUDT gold NPs (2 mg) and approximately 10-fold excess of SC (0.04
mg, 0.17 pmol) was dissolved in dichloromethane (200 p.L) and allowed to react for 10 minutes. After the reaction, the mixture was cleaned using acetone and NPs were
63

extracted. Then NPs were redissolved into dichloromethane for AUC measurement. For
TEM measurement, the reacted solution was filtered through Sephadex column, separated
NPs that went through the column to NPs that stayed in the column. The NPs that went through the column were extracted using acetone, and redissolved into dichloromethane.
The NPs-Sephadex complex was soaked in excess amounts of dichloromethane while waiting for the Sephadex to sediment and extract NPs in the solution on the top in order to prepare the TEM sample.
3.6.5. AUC
The sedimentation velocity experiments for both dimerized and non-dimerized
NPs were performed using a Beckman Optical XL-A outfitted with the An-60 Ti rotor and scanning absorbance optics, in 12-mm path length double sector centerpieces with sapphire windows. The NPs were sedimented in dichloromethane at 15,000 r.p.m. at
20 'C. Data from over 50 scans were chosen to be representative of the whole run (radial step size of 0.003 cm). The raw AUC data were analyzed with SEDFIT, an open-source computational software package available free online.
3.6.6. TEM
TEM images were obtained using JEOL 200 and JEOL 2010 instrument. The
TEM grids were placed on the Kim-Wipe, and 1-2 drops of the solution (~ 1-2 ptL) were dropped on the grid and allowed it dry slowly. Self-assembled structure and dimers of
Janus NPs were analyzed using ImageJ software. The structure is considered as selfassembled structure or dimers only when the inter-particle distance is less than maximum stretched length of two ligands and the linker.
64

3.6.7. Gel electrophoresis
Gel electrophoresis was performed using 3 wt% agarose gel under fixed voltage
(45 V). 10% TAE buffer solution is diluted to 1% by adding milliQ water. Then, agarose
(3 g) was added in 1 % TAE buffer solution (100 mL). The mixture is heated using microwave for about 2 minutes until the solution becomes transparent. The solution is poured into the gel holder, removed air bubbles, and let it cool for an hour. NPs (2 mg) were dissolved in milliQ water (1 mL), and the solution (- 40 pL) was injected to each well. The gel was placed under 45 V for 4 hours. After separation, the gel surface was cut using razor blade, and placed TEM grid inside, and voltage applied for additional 5 minutes. Then, TEM grids were dried in air covered with a laboratory dish overnight before TEM analysis.
65

1. Singh, C.; Ghorai, P. K.; Horsch, M. A.; Jackson, A. M.; Larson, R. G.;
Stellacci, F.; Glotzer, S. C., Entropy-mediated patterning of surfactant-coated nanoparticles and surfaces. Physical Review Letters 2007, 99 (22), -.
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C.; Christianen, P. C. M.;
Sommerdijk, N. A. J. M.; Meskers, S. C. J.; Schenning, A. P. H. J., Disk micelles from amphiphilic Janus gold nanoparticles. Chemical Communications 2008, (6), 697-699.
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66

nanoparticles decorated with a mixed shell of oligo(p-phenylene vinylene)s and ethyleneoxide ligands.J. Mater. Chem. 2008, 18 (29), 3438-3441.
16. Vilain, C.; Goettmann, F.; Moores, A.; Le Floch, P.; Sanchez, C., Study of metal nanoparticles stabilised by mixed ligand shell: a striking blue shift of the surface-plasmon band evidencing the formation of Janus nanoparticles.J. Mater.
Chem. 2007, 17 (33), 3509-3514.
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18. Wang, B.; Li, B.; Zhao, B.; Li, C. Y., Amphiphilic Janus gold nanoparticles via combining "Solid-State Grafting-to" and "Grafting-from" methods. Journal of the
American Chemical Society 2008, 130 (35), 11594-11595.
19. Pradhan, S.; Xu, L. P.; Chen, S. W., Janus nanoparticles by interfacial engineering. Advanced Functional Materials 2007, 17 (14), 2385-2392.
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21. Lattuada, M.; Hatton, T. A., Preparation and controlled self-assembly of janus magnetic nanoparticles.Journal Of The American Chemical Society 2007,129
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25. Isojima, T.; Lattuada, M.; Vander Sande, J. B.; Hatton, T. A., Reversible clustering of pH- and temperature-responsive Janus magnetic nanoparticles. Acs
Nano 2008,2 (9), 1799-1806.
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Marder, S. R.; Perry, J. W., Ultrabright supramolecular beacons based on the selfassembly of two-photon chromophores on metal nanoparticles. Journal of the
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28. Zheng, N.; Fan, J.; Stucky, G. D., One-step one-phase synthesis of monodisperse noble-metallic nanoparticles and their colloidal crystals. JOURNAL OF
THE SOCIETY 2006, 128 (20),
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Bakr, 0. M., Determination of nanoparticle size distribution together with density or molecular weight by 2D analytical ultracentrifugation. Nature Communications 2011,
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33. Walther, A.; Hoffmann, M.; Muller, A. H. E., Emulsion polymerization using Janus particles as stabilizers. Angewandte Chemie-International Edition 2008,
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34. Sacanna, S.; Kegel, W. K.; Philipse, A. P., Thermodynamically stable pickering emulsions. Physical Review Letters 2007, 98 (15), 158301.
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68

When two dislike thiols self-assemble on the gold nanoparticle (NP) surface, they can separate into stripe-like domains. The striped surface provides unique surface properties to the
NPs. One example of these properties is divalency. A particle coated with stripe-like domains implies two defect points at the poles of NPs. We have shown that these two polar defects can be selectively functionalized with molecules that in turn can act as handles for further assemblies.
With this approach we can make chains of the divalent NPs and create 1 -D assembly of NPs.
Here, we apply a recently developed theory on the gelation of divalent nanoparticles 1 to develop functional 3-D network of NPs. In this chapter, we will show NP gels consisting of water-soluble striped NPs. The network structure is formed by self-assembly of divalent and some multivalent
NPs when they are mixed with divalent metal ions. Gelation occurs only using ionic interaction between NPs, and it requires both divalent anionic NPs and divalent cations. Gels are investigated to determine their properties using rheological characterization. We found a gel at room temperature when the concentration of NPs is above 38 wt%. This gel is stable at temperature up to 70'C, and the gel behavior does not change after multiple heating-cooling cycles.
69

A gel is a solid material that has 3-D network structure. Gels are defined as dilute crosslinked system which show no flow under steady-state.
2 Mechanically, it is defined as a material that exhibits a larger storage modulus (G') than loss modulus (G") (G'> G") and the both moduli are independent of the broad spectrum of frequency (typically 10-' to 102 Hz) 3
(Figure 4.1.)
Originally, gels were synthesized using polymeric materials only, but recently, many studies were done using polymer-particle composite gel, colloidal or nanoparticle gels.
Gels can be made either by chemical linking or physical linking of materials. Chemical gels form covalent bonds, and physical gels form non-covalent, reversible bonds using secondary interaction such as ionic interactions, hydrogen bonds and hydrophobic interactions.
equlbrium modulus slopemwo
log 60
Figure 4. 1 The schematic graph represents the mechanical property of a generic gels. The
G' and G" are independent of frequency, and G' is larger than G" (reprinted from ref. 3)
70

Gels can be classified into strong gels and weak gels. Both strong gels and weak gels show the same mechanical behavior under frequency sweep as described above. However, they behave differently under wide range of strain. Strong gels remain solid at relatively large strain under the strain sweep test, while weak gels break (i.e. G" becomes larger than G') when high strain is applied. In addition, when strong gels ruptured, they do not recover to the original state until they are melted or reset. On the other hand, weak gels "heal" after failure when the sufficient time is given. Chemical gels and some physical gels are strong gels, and most of physical gels result in weak gel.
Our NPs gels are in the category of physical gel, since NPs self-assemble into gels via ionic interactions. Therefore, recent development of physical gel of various materials would be reviewed below. Especially, we focus on the gelation of colloidal particles or nanomaterials using specific interactions, since that would be the most similar to our system.
4.1.2. Physical Gels using Polymeric Materials
Many polymer-based physical gels are developed in past decade. Some use only polymers, others use the composite of polymer and inorganic materials (e.g. clay, nanoparticles etc.).
For gelation using colloidal polymeric particles, Wang et. al. haven shown the moldable tissue engineering scaffold made of two different Poly(D,L-lactic-co-glycolic acid) (PLGA).
4
PLGA is FDA-approved biocompatible, and biodegradable materials, which has been reported widely its use as tissue engineering scaffolds.
5
'
6
Here, oppositely charged PLGA NPs were prepared by solvent diffusion method. One batch of NPs was coated with polyvinylamine
(PVAm) and the other batch was coated with poly(ethylene-co-maleic acid) (PEMA) Positively
71

charged PLGA-PVAm NPs were mixed with negatively charged PLGA-PEMA NPs to form a colloidal gel. The authors measured its viscosity and shear thinning behavior by varying the mixing ratio of two particles as well as by controlling the total concentration of particles in suspension. The network deforms with shear and recovers back to the gel state, showing its possibility toward injectable or moldable tissue engineering scaffolds.
Aida and co-workers developed high-water-content moldable hydrogels by mixing clay and a dendritic polymer.
7
They generated the gel in two steps. First, clay nanosheets (CNSs) are exfoliated with sodium polyacrylate (ASAP). Then, dendritic binders containing multiple guanidium ion pendants were mixed to form a gel. (Figure 4.2) Only small amount of clay (2-3 %
by mass) and dendritic binder (< 0.4% by mass) are needed to form a gel in water. This shaperesistant, free-standing gel shows superior mechanical properties with G'-0.5MPa, when ASAP exfoliate CNSs. The gel shows rapid recovery after a large-amplitude oscillatory break down
(Figure 4.2 (e)), and its shape is unchanged when immersed in THF (Figure 4.2(d)), salty water, and weakly acidic/basic solutions.
72

(b) (a) aa__ a a
(c) v~Y
(d)
1x l0
[CNS]
5.0%..
4.0%
* .
3.0%.
2.0%.
......... e~
~
1 x 10'
1 x 103
0.01
0.1 1
Frequency (rad s-
1
)
10
0 ee -
0
100
(e)1
: OM
1 x 0
0 1 X 104 e 2::
1 x 103
0 1,000 2,000
Time (s)
3,000 4,000
Figure 4. 2 The hydrogel made with clay nanosheets and dendritic binder, (a) ASAP (red) exfoliates clay nanosheets and add negative charges on the clay surface. Then, dentritic binder containing mutiple guanidium ions (blue) are added to form hydrogel; (b,c) Hydrogels are moldable into various shapes; (d) G' value increases as the amount of clay nanosheets is increased;
(e) The gel quickly reforms after large deformation. (reprinted from ref. 7 )
73

(a)
~
(b) qz~
+4FF
WOH r
L
RR
H
HEC-Np
R u -H, -CH
2
CH
2
OH
Ol
N
H
OH
1
PVA.MV
O O
HPVA-M
HN
0
~O__1NQd_7N-
SF
4
Figure 4. 3 Supramolecular hydrogel using CB[8] (a) Mechanism of supramolecular assembly using CB[8]. CB18] can bind with two different guests, one is electro-deficient molecule, and the other is electron-rich molecule. The equilibrium is towards the formation of complex so that it can be used for stable gel formation; (b) The formation of hydrogel using C181. Two different polymers each grafting the guest molecules on the side chain mixed together with C18] leads to high-water content hydrogels. (reprinted from ref. 8 and 9)
Besides ionic interaction, supramolecular approach is also used to form a physical gel.10
Scherman and co-workers have shown the gelation using cucurbit[n]urils (CB[n], n= 5-8 of
74

glycouril units)."'
9 CB[n] molecules are barrel-shaped macrocyclic hosts, which are capable of forming inclusion complexes with appropriately sized guest compounds in water with high affinity. Among CB[n] molecules, CB[8] can host two molecules. 12 (Figure 4.3(a)) The first guest molecule would be an electron-deficient molecule, such as viologen (MV) and the second guest molecule would be an electron-rich molecule such as 2-naphthol. A stable ternary complex is formed when CB [8] is mixed with two guest molecules in the ratio of 1:1:1. They used this concept to prepare cross-linked polymer networks. They synthesized two polymers, each polymer has one type of the guest molecule on the side chain. (Figure 4.3(b)) When two polymers are mixed with CB[8], self-assembled hydrogel with very high content (up to 99.7%) is formed. This system has tunable mechanical property by vatying the content of guest molecules, which shows its versatility for industrial development as well as understanding for the structureproperty relationship of supramolecular hydrogels.
4.1.3. Physical Gels using Inorganic/Metallic Materials
Gels that are composed of graphene oxides
13, 14, semiconductor NPs
15-18, and noble metal NPs 19-21 are developed recently.
Graphene/iron oxide hydrogels are made using self-assembly of graphene oxide (GO) sheets and in situ deposition of iron oxide NPs on the GO sheets by ferrous ions as a reducing agent the reduce the GO sheets.' 3 This process was done in mild condition, and the composition of gels was changed by adjusting initial pH as illustrated in Figure 4.4. At low pH, graphene/a-
FeOOH hydrogels are made, and graphene/Fe
3
0
4 hydrogels are made at high pH. The hydrogels are freeze-dried and aerogels are made. Graphene/Fe
3
0
4 hydrogels/aerogels showed magnetic behavior. Graphene/a-FeOOH aerogels could uptake gasoline selectively in water, and hydrogels
75

could adsorb Cr(VI), and Pb(II) in water. This indicates that these hydrogels/aerogels could be used in industrial water purification. When oil-adsorbed Graphene/a-FeOOH aerogels are burned, free-standing a-Fe
2
O
3 monolith can be prepared as well.
Assembly
Fe(I) i) Graph enekL-FeOOH
1@ *~
~N'~'
@00
ii) GrapheledFeO
4
Figure 4. 4 The formation of graphene oxide/iron oxide hydrogels in different pH condition
(reprinted from ref. 13)
Eychmtiller and co-workers have done many works to make hydrogels and aerogels using semiconductor NPs and noble metal NPs.' 5,18-20,22 In their earlier work' 5
'19, they made hydrogels of CdTe NPs and noble metal (Au, Ag, Pt) NPs by destabilizing the colloidal solution using ethanol or hydrogen peroxide. Then, they used liquid CO
2 for supercritical drying that results into aerogels. Although this method was able to create hydrogels and aerogels, the process was very slow, so it took long time (weeks to months) to form a gel. In addition, in the case of monometallic aerogels from gold, silver, and platinum, the authors report variations in the reproducibility due to difference in the concentrations of the components in the colloidal solutions and environmental condition change. This method is not using the direct assembly or interaction between NPs, therefore it is difficult to control the structure of gels.
76

The same group has adapted the new method to make hydrogels/aerogels by using
5mercaptomethyltetrazole as the ligand coat for CdTe and gold NPs.
17, 18
Two 5mercaptomethyltetrazole molecules at the different NPs coordinate with two Ca 2 ions and the gelation would occur. (Figure 4.5. left) This method turned out to be faster and more reproducible method compared to the method explained above. They successfully showed CdTe
NPs hydrogels 18 and CdTe/gold NPs composite hydrogels.
17
CdT.
NO.
CdT1NO Cd(OAC)h
H
H,
CH,
H
NO
2
1
H20
CdT* NO
Figure 4. 5 CdTe nanoparticle hydrogel/aerogel using 5-mercaptomethyltetrazole (left) The mechanism proposed for the gelation of CdTe NPs covered with 5-mercaptomethyltetrazole (right)
TEM and HR-TEM image of dried hydrogel fragment. Inset: a true-color image of a CdTe hydrogel under daylight (left) and under UV (X = 365 nm) (right). (reprinted from ref. 18)
77

Theoretical calculations for the self-assembly behavior of patchy particles has been recently published.
1,23
This study used the particles with discrete number of attractive interaction sites (M). They found that when divalent (M=2) and trivalent (M=3) NPs are mixed, at a certain volume fraction (po 3 ) of NPs, the gelation of particles occurs at above a given temperature, where percolation is achieved. (Figure 4.6) At percolation, a significant number of
NP clusters of above a certain size appear. Below percolation (i.e. higher temperature), NPs aggregate only as smaller finite-size clusters, and above percolation (i.e. lower temperature), NPs mostly belong to a network of theoretically infinite size. It should be noted that only small amount of trivalent NPs are needed to form a gel. For the volume fraction p03 =0.04 that is shown in Figure 4.6, the ratio between the divalent NPs to the trivalent NPs is 17 to 1. Therefore, we can predict that the gelation would occur when the amount of NPs with more than two defects is more than 5.5 % in the system. This NP solution to gel transition is thermally reversible, and transition temperature can be tuned by varying the amount of trivalent NPs, and the volume fraction of NPs. When more trivalent NPs are added to the system, the percolating temperature becomes higher since there is more cross-linking points in the system. Also, when the concentration of NPs is increased, the percolation temperature increases. The phase diagram of the system is shown in Figure 4.7.
78

AV, i
T=0.065
(at percolation)
I
'see
T=0.09
(below percolation)
Pb < pl
T=O.AS
(above percolation)
Pb >> Pb
I ~ s.i4~~
Figure 4. 6 The theoretical calculation of nanoparticle gel Representation of the studied system for three different temperatures (in units of uo) at density pa3=0.04. The pictures show the structure of the system below percolation (left), where particles are aggregated in small finite clusters, at percolation (center), where a spanning cluster first appears, and well above percolation
(right), where all of the particles belong to the infinite cluster. (reprinted from ref. 1)
79

0.1
-A
0.09
A A AA
0.08
H.-
0.07
A
A
0.06
0.05
A A A
A A A
A - A.
-~777,I..
I I I
0.01
3
A
A
A
*OleU
U-1 a.
Aa a
U
U
0.1
Figure 4. 7 The phase diagram of the mixture of divalent and trivalent particles. The black triangle represents the liquid state (above percolation), and the red square represents the gel state.
The gel-to-liquid transition temperature is shown in red dot lines, and it indicates the temperature increases as the volume fraction of NPs (p&) increases. The green line shows the phase separation region, where the gel and the solvent phases are separated. (reprinted from ref. 1)
There are some studies showing the assembly by creating the attractive interaction sites on the surfaces of NPs.
7 ' 18'24 Nonetheless, the 3-D assembly of gold NPs with precise control of interaction sites on the surface that leads to the gelation of the whole system has not been reported yet. Here, we describe a new method to create NP hydrogels and their properties. This method easily controls the structure and is fast in making NP hydrogels. In addition, it is also versatile that we can study further to develop functional materials.
80

Based on the theory explained in section 4.2, the gelation of the divalent NPs using the ionic interaction between NPs is designed. When two poles of the striped NPs are functionalized via place-exchange to negatively charged ligands, e.g. carboxylic acid terminated thiols, the NPs can act as the divalent anions and they attract each other and form a gel when divalent metal cations are added.
In order to test the hypothesis, we first used the striped gold NPs coated with 1nonanethiol (NT) and 6-mercapto-1-hexanol (MHol). We synthesized NT/MHol NPs by modified Stucky synthesis. NT/MHol NPs are ethanol soluble, so the place-exchange reaction of
NPs using 11 -mercaptoundecanoic acid (MUA) was done in ethanol for 20 minutes. After 20 min, acetone was added to the solution and collected precipitated the gold NPs. The gelation was tested in the 1:1 mixture of ethanol and MilliQ water as the solvent, because NT/MHol NPs were partially soluble in water. We made the concentrated NPs solution of 100 mg/mL by mixing 100 mg of NT/MHol NPs and 1 mL of the solvent mixture. When NPs are completely dissolved in the solvent mixture, 1 piL of 0.002 M aqueous solution of CaCl
2
'. 2H
2
0 was added. The samples were placed in freezer for several hours for gelation. Small amount (1-2 mg) of all MUA nanoparticles was used as a crosslinker to some samples for comparision.
When the concentration of nanoparticle was ~100 mg/mL, none of the samples was turned to gel in the freezer. After evaporation of solvents, samples that contains Ca
2
+ and/or all-
MUA NPs (crosslinker) turned to gels in the freezer. (Figure 4.8) The samples without any crosslinker showed the most stable gel structure at given temperature. (Figure 4.8, sample 3)
81

HOOC COOH +
Water/EtOH
Bifunctionalized nps Homoligand protected nps with with MUA MUA (Crosslinker)
MUA
2
4
tncMona with MUA
Yes
Yes
Yes
Yes
2
Yes
Yes
No
No
Yes
No
Yes
No
Yes
No
Yes
No
4
Figure 4. 8 Nanoparticle gel using NT/MHol nanoparticles. Schematic reaction is shown on the top. Bottom: list of samples tested for preliminary result, and nanoparticle gel of sample 3. The stable gel is maintained when the glass slide is tilted.
This preliminary result showed the gelation at very low temperature, but it has some drawbacks. NT/MHol NPs were not completely dissolved in water, so it was hard to make the concentration of NP solution larger than 100 mg/mL. As described in section 4.2, the gel-toliquid transition temperature becomes higher as the concentration of NP solution increases. In addition, adding ethanol to the sample was not desired to make complete hydrogel. Ethanol tends to evaporate quickly compared to water, therefore, the concentration of the sample changes as time goes by. These drawbacks lead us to design a new way to make the divalent NP hydrogel.
4.4.
To overcome the problems of preliminary results described above, we decided to design a new method. Following conditions are needed for gold NPs to make the hydrogel using ionic interaction between divalent gold NPs:
82

1) The NPs should be soluble in water.
2) The ligands on the NPs should not be charged in order to minimize the non-specific interaction between divalent nanoparticles.
3) The ligand combination should have a strong driving force toward phase separation and make the striped domain on the NPs' surface.
We selected the NPs coated with the mixed ligand shell of 1-hexanethiol (HT) and 2- [2-
(1-mercaptoundec-6-yloxy)-ethoxy]-ethoxy
-ethanol (EG3) which meet the conditions described above. The molar ratio of HT to EG3 is 1:3 to ensure the solubility in water. The both HT and
EG3 molecules are not charged at ambient pH, so there would be less interaction between ligand shell and metal cations that we would add for gelation. Our group recently investigated that the striped HT/EG3 NPs selectively capture some cations.25 From the study, we found that HT/EG3
NPs do not capture both Na+ and Ca
2
+ ions. Therefore, we selected to use Na+ for the polefunctionalization and Ca
2
+ for the gelation, assuming that there would be no interaction between
EG3 ligands and these cations. The size difference between HT and EG3 ligands enable to form striped domain on the nanoparticles in the core size range of 4-6 nm. We synthesized HT/EG3 nanoparticles of 5-6 nm (core size) to make sure that the majority of the nanoparticles are the striped nanoparticles.
The gelation of the striped nanoparticles involves two steps: first, the pole functionalization of the striped NPs to create the divalent NPs, and second, the gelation using ionic interaction between the divalent NPs. (Figure 4.9)
83

1. Pole functionalization
O OH
+ HS
S
MUA
2. Gelation
HO
SO
_N _-11SN.
~Na-Odefects
HO -/O-
O S s
OH
Na,
O
-0 _\ H
+
SHO
0 pH=7-8
OH
NaOH (aq)
/DMSO
H0
0
O
*Na-O
HO --
_S
O 0
Some multivalent
2
+ +
NPs due to the other than poles
NaOH (aq)
S
S
S
S
S
O~O
0~
OH
0- Na* o
0
ONOH
) 41 1)
I
:01
Figure 4. 9 The gelation of the HT/EG3 striped NPs.
First, a pole-functionalization is done by reacting HT/EG3 NPs with 11mercaptoundecanoic acid (MUA). The 7:3 mixture of 10 mM NaOH solution and dimethylsulfoxide (DMSO) was used for the solvent to dissolve MUA. The reaction performed under slightly alkali condition (pH=7-8) using 10 mM NaOH solution to make sure all carboxylic acids (-COOH) group on MUA is ionized (-COO-). We have found that if Na+ is not present, -COOH groups on MUA form hydrogen bond after pole-functionalization, and it is impossible to re-dissolve NPs after drying NPs. 100 mg of NPs are dissolved in 3 mL of MilliQ water, and the small excess MUA (~ 0-fold in molar excess) is dissolved in 3 mL of the solvent mixture and added to NPs solution. The reaction takes place for a short period time (- 30 minutes) and it is quenched by adding tetrahydrofuran (THF). From this step, we can obtain the divalent NPs precipitate on the bottom and unreacted MUA would be dissolved in THF. After cleaning out MUA via multiple centrifugation of NPs in THF, the NPs are fully dried in a dessicator and made into powder for the further step.
84

The second step is making a hydrogel using the divalent NPs and Ca 2
+ ions. For the desired concentration, the divalent NPs was dissolved in aqueous 10 mM NaOH solution. Then
Ca2+ ions were added using 0.1 M CaCl
2
H
2
O solution. The ratio between the amounts of NPs to that of Ca2+ ions was fixed for all tested concentrations. It should be noted that we made a gel without adding other type of NPs. It indicates there are some non-divalent NPs present in our
NPs batches. The result was reproducible between several batches with similar size distribution.
Ca2+ ions are added in excess of ~ 1.5-fold compared to the calculated molar amount of MUA molecules on the divalent NPs. Na+ ions are added in excess of~ 2-fold compared to the calculated molar amount of MUA molecules on the divalent NPs.
4.5.
Samples with different NP concentrations (9 wt%, 23 wt%, 38 wt%, 47 wt%, 55 wt%) were prepared to form a gel. Other conditions, such as the amount of Na+ and Ca2+ ions in the solution and the volume of solvent, were kept constant between samples. We found a solid gel at room temperature when the concentration is higher than 38 wt%. The sample of 38 wt% NPs contains 600 mg of pole-functionalized NPs dissolved in 1 mL of aqueous solvent (735 tL of 10 mM NaOH (aq) and 265 [tL of 0.1 M CaCl
2
-H
2
O(aq)). When the NPs are dissolved to NaOH solution, they showed good solubility and NPs solution flowed like liquid. However, when Ca2+ ion is added to the 38 wt% sample, it instantly forms a soft solid structure at room temperature
(20 'C). The gel was very soft and it was hard to pick up the whole sample. However, the small portion of the sample can be picked up. When the small amount of sample is held between tweezers, it deforms easily and elongate to some extent when it is pulled from the ends. (Figure
4.10) The small portions of the sample adhere together to make larger sample showing that the
85

gel has self-healing property. The gelation is concentration dependent at given temperature, and the gel breaks when Na+ was added. It is thought that when large amount monovalent cation is added, ionic interaction of two MUA and one Ca2+ ion is replaced to that of one MUA and one
Na+ ion. Therefore, the gel breaks and it becomes its solution state. It shows that the divalent cation holds the network structure together and create the gel as we assumed in section 4.3. No visible difference between 38 wt% and 47 wt% was found, and it was confirmed by rheology measurements that is discussed at following section.
Figure 4. 10 The hydrogel formed by the striped NPs. The gel is formed at room temperature when concentration of NPs is 38 wt% or above. The fragment of the gel can be picked up using tweezers.
Lower concentration samples (9 wt%, 23 wt%) were solution at room temperature after
Ca 2
+ ions are added. However, this solution showed different flowing behavior compared to the control that did not contain Ca 2
+. The sample containing Ca 2
+ ions flows in a lump, whereas the control that does not contain Ca2+ runs like water leaving a long tail behind. At this point, we do not have enough data to understand whether this is a consequence of a difference in rheological
86

or wetting properties. We put the sample in the refrigerator in order to find out whether gelation takes place at lower temperature. The sample did not show visible gelation when the temperature is lowered to 4 'C.
4.10):
To prove the origin of the gel we analyzed the following samples as controls (Figure
1) HT/EG3 NPs without pole-functionalization is dissolved in NaOH (aq) solution and
Ca
2 + is added. (no MUA, Ca 2 )
2) The pole-functionalized particle is dissolved in NaOH (aq) solution, and no Ca
2
+ is added. (MUA, no Ca )
3) EG3 homoligand coated NPs is synthesized and is dissolved in NaOH (aq) and Ca
2 added. (no striped structure, no MUA, Ca 2 )
All controls were made in concentration of 47 wt% in order to compare with the sample of the same concentration. The sample of 47 wt% became a gel at room temperature after adding
Ca
2
+ ions.
First control was designed to verify the effect of divalent NPs on gelation. HT/EG3 NPs were used as synthesized without pole-functionalization. The second step to induce gelation was done as described in section 4.4. The gelation did not occur at room temperature after adding
Ca> ions. (Figure 4.11 (1)) It showed that MUA on the gold NPs plays important role in gelation.
The second control was made to show the effect of Ca2 ions. The same NPs that are pole-functionalized using place-exchange reaction described in section 4.4 were used. In gelation
87

step, NPs were dissolved in the same amount of NaOH (aq) solution, and water was added instead of CaCl
2
*2H
2
0 (aq) solution. When the divalent NPs were dissolved in water without
Ca+, no gelation occurred. (Figure 4.11 (2)) The result showed that divalent cations interact with
MUA and the specific interaction enabled to undergo gelation of gold NPs.
1)
HON-O
0OOH os + Ca
2 NaOH (ag) o0 S S
HO--'O
2)
HO'N-O s10O
0
'Na .0-
O'
HO -/
0
3)
HO'\-O
HO-S -0
H
O
-~\ -,
S s-
S-)l s
S
_/
ON-OH
-/'OH
O- Na+
0 -OH
(aq)
-/OH
-0 _ -/-OH
0 ' 0 + C a
2
+
N a O H (a g )
S
HO-'O
0 -OH
Figure 4. 11 Control experiments were performed using (1) HT/EG3 gold NPs and Ca2+, (2) pole-functionalized HT/EG3 gold NPs without Ca
2
+,
(3) homoligand EG3 coated NPs and Ca
2
+.
All
control samples did not show gelation at the concentration of 47 wt%.
The last control was made using NPs coated with only EG3 ligand. This homoligand covered NPs were synthesized using one phase synthesis in ethanol 26.
The detailed experiment
88

conditions were described in section 4.7. Homoligand NPs were used to investigate the effect of stripe-like surface structure of NPs, and the non-specific interaction between EG3 molecule and
Ca2+ ions. EG3 NPs were dissolved in NaOH (aq) solution, and Ca2+ ions were added using
CaCl
2
*2H
2
0 (aq) solution as described in section 4.3. The solution did not undergo gelation at room temperature. This indicates that non-specific interaction between hydrophilic ligand and ions is negligible in gelation of NPs.
In addition to three controls that described above, we are planning to make another control. It is made using EG3 homoligand covered gold NPs that contain MUA via placeexchange reaction. The place-exchange reaction was done as same as pole-functionalization described in section 4.4. Then, the gelation will be tested using Ca2+ as described above. This would confirm the role of MUA and Ca2+ ions and the stripe-like surface structure.
All three controls made flowing liquid samples, and no solid structure was found in the temperature range tested (5-50 'C). In addition, they showed no difference in viscosity when observed by naked eyes. We also tested homoligand MUA covered gold NPs, and when Ca2+ ions was added to the NP solution, NPs precipitated on the bottom of vial rather than making a solid structure containing water inside. This precipitation goes back to solution when excess amount of Na+ ions was added, showing that the gelation is sensitive to types of cation as well.
This indicates that divalent NPs containing MUA on the surface as well as Ca2+ are important to create hydrogels.
89

Rheology measurements were performed on the samples and controls described in section 4.5. As described in section 4.1.1, the mechanical property of gels can be determined by frequency sweep and amplitude sweep. The frequency sweep of the nanoparticle gel in different concentrations is shown in Figure 4.12(a). It is performed using 4 mm parallel plates at 10 'C and under the constant strain of 0.3%. When the concentration of the sample was higher than 38 wt%, the storage modulus (G') was larger than the loss modulus (G") for the frequency range that we tested. The storage modulus of the strongest gel (47 wt%) we tested was between 400 -
500 Pa, which is in the range of typical supramolecular hydrogels
0
. In addition, from amplitude sweep, G' and G" decreases as strain increases, and the transition between the gel and the quasiliquid occurs when strain is
-
5%. (Figure 4.12(b)) This transition indicates that the gel is the weak gel.
When the concentration was low (23 wt%), it was liquid at room temperature (20 'C).
However, when frequency sweep is performed at 10 'C, G' is slightly larger than G" and it breaks at very low frequency.
90

1000
0 cc
100
CU 10
r
F : " : " :'
1
:1
U, 1
~
U
0.1
7N all
U.~~~ U
~~
PUU
0.01
1 E-3
1E-4
0 .1
lol
1
, .
I A t
10
Angular Frequency (rad/s)
100
(C) 1000
100
0
********
10
1
0.1
0.01
1 E-3 r r .. ..
.
20
0 ----
40
TemDerature(*C)
* G'-heating2
G"-heating2
* G'-cooling2
G"-cooling2
60
(b) jo
0
102
100
10
.5
V 1
U...
"r.
000-0
1
Strain (%)
10
0-00
0000
0000.
100
SG'
G*
0.1
I
0
I
50
.
I
100
Time (s)
*
I
150
*
I
200
Figure 4. 12 The rheology measurements of NP gels. (a) Frequency sweep of different NPs gel samples, 47 wt% (red), 38 wt% (green), 23 wt% (blue), and control of 47 wt%. Filled square is
G' and open circle is G". (b) Amplitude sweep of 47 wt% sample. It shows a property of a weak gel.
(c) Temperature sweep of 38 wt% gel sample (red and blue) and the control (black and green). (d)
Thixotropy measurement of 38 wt% sample. The gel is recovered immediately when large strain is removed.
Temperature sweep measurement also performed for 38 wt% gel. (Figure 4.12(c)) For 38 wt% sample, though the difference between G' and G" value gets smaller above 60 'C, no clear transition between gel-to-liquid was found as predicted in theory for tested temperature range
91

(10 'C-70 'C). It is because that the gel is formed at very high volume fraction of NPs. The sample of 38 wt% would be approximately pT3~0. 19 according to the theory.' In theory, when the volume fraction increases, the bond probability (Pb) converge to 1 regardless of temperature.
Therefore, the volume fraction of 0.19 may be high enough to have the critical bond probability even at the highest temperature that we tested. The gel remains stable, and very small hysteresis was shown for both the gel sample and the control (divalent NP without Ca2+, control 2).
Thixotropy was also measured by applying oscillating strain forces. (Figure 4.12(d))
When large strain is applied, the gel breaks (G'<G"), however, it immediately recovers its original state (G'>G") when strain is removed. This shows that our material has 'self-healing' property.
We tried to construct the phase diagram of gelation similar to that shown in Figure 4.7.
We have converted our sample concentration to approximate volume fraction of NPs. Our group has known experimentally that NPs with core size ~ 5.5 nm coated with the ligand length ~1 nm, would have the density of 6 g/cm 3 . Using this number, we found that the stable gel of 38 wt% has the volume fraction of approximately 10 %. The volume fraction in the theory paper I was
p&T, which p was the number density of particle in the system, cY was the diameter of a particle. If we convert 1.0% volume fraction into p&3, we can obtain 0.19.
In the phase diagram, there was a region that the gel and water phase separate. (below the green line with purple circles in Figure 4.13) In order to investigate such state exists, we added the excess amount of water to the gel sample (pa
3~0.006), and put it in the freezer to lower the
92

temperature. We also put the divalent NPs solution without Ca
2
+ ions as a control. When the system is completely frozen, the difference between the sample containing Ca
2
+ and the control without it was visible. The gel sample phase separate from aqueous solvent, and it goes back to homogeneous solution (liquid state: black triangle in Figure 4.13) when temperature increased.
On the other hand, the control sample did not phase separate, and the whole solution was homogeneously frozen.
0.IA
0.09
A
08 -A
A
I
A A
A A A
A ...
A
A
A-'
A
A -
I
A
A
A
-A
A
A
Gel state
3
gel at 20
2 +
2
1
(-
3 =0.006
3
Figure 4. 13 Reconstructed phase-diagram of the divalent NP gels. We found that when pa
3
=0.19, the gelation occurs below 20 IC. When p&l=0.005-0.006, the sample showed phase separation from water at -4 OC. The same particles without Ca2+ did not show such separation.
When temperature increases, the phase-separated sample went back to homogeneous liquid state showing such phase separation is reversible.
93

In summary, we obtained a stable gel composed of divalent gold NPs. When the concentration of NPs is higher than 38 wt%, the stable gel is formed at room temperature. These gels remained stable above the room temperature up to 70 'C in our experiment. The gel quickly recovers its original shape after deformation.
Though this method is a new method that can obtain controlled noble metal NP gels, it has a drawback that the gel requires very high concentration of NPs. However, this drawback can be overcome because of versatility of this method. For example, we can try to reduce the concentration of NPs and form a gel either by adding trivalent cation (e.g. Fe 3 ) or by introducing the stronger metal binding ligand on the NP. In addition, we can expand our system to other noble metal NPs coated with two different ligands and try to create other monometallic
NP gels or bimetallic NP gels by mixing two NPs with different core metals.
All chemicals are purchased and used as is unless specified. Gold(III) chloride trihydrate
(HAuCl
4
3H
2
0), Chloro(triphenylphosphine)gold(I) ([(C6H5)3P]AuCl), 1-Hexanethiol (HT),
11 -Mercaptoundecanoic acid (MUA), tert-butylamine-borane complex, sodium borohydride
(NaBH
4
) are purchased from Sigma-Aldrich, and used as received. 2-[2-(1-mercaptoundec-6yloxy)-ethoxy]-ethoxy}-ethanol (EG3) are purchased from ProChimia Surfaces, and used as received. MilliQ water (EMD Millipore) was used when water is needed. Carbon coated TEM grids were purchased from EMS Acquisition Corp.
94

4.9.2. Synthesis of gold nanoparticles
Gold NPs coated with 1:3 ratio of HT and EG3 are synthesized using a modified Stucky synthesis.
27
The mixture of solvents in ratio of 1:1 chloroform (CHCl3):dimethylformamide
(DMF) was used. [(C6H5)3P]AuCI (223.06 mg, 0.45 mmol) was dissolved in solvent mixture
(20 mL). The 1:3 mixture of thiols was added (HT, 13.0 mg; EG3, 89.90 mg; 0.45 mmols in total) and stirred for 20 minutes. Then, tert-butylamine-borane complex (217.43 mg, 2.5 mmol) was dissolved in solvent mixture (20 mL) and temperature is increased to 55 'C. The solution slowly becomes yellowish brown, then turns to darker when the reducing agent is added. The reaction took place for 1 hour at 55 'C, and then for 30 min at room temperature (20 C). After the reaction is done, the solution is placed in the refrigerator for overnight precipitation. The precipitation collected using centrifuge, and washed several times using chloroform. Particle size distribution is measured by TEM images.
Gold NPs coated with EG3 are synthesized using one-phase synthesis in ethanol.
26
HAuCl
4
3H
2
0 (177.2 mg, 0.45 mmol) was dissolved in ethanol (100 mL). 0.45 mmol of EG3 was added in the solution, and stirred for 20 minutes. Then, NaBH
4
(189.1 mg, 5 mmol) was dissolved ethanol (100 mL) and it was slowly added drop-wise using a syringe needle. Upon addition, the gold-thiol solution became turbid yellowish-green and slowly darkened to purplishblack. After the complete addition of NaBH
4 solution, the solution was stirred for 3 hours. When the reaction is done, it is placed in the refrigerator for overnight precipitation. The precipitation is collected using centrifuge, and washed several times using ethanol, water/diethyl ether, and tetrahydrofuran (THF). Particle size distribution is measured by TEM images.
95

Pole-functionalization of NPs is done using the similar method that our group previously has done.
28
100 mg of NPs was dissolved in water (3 mL), and ~ 10-fold excess amount of
MUA (4.8 mg), which dissolved in 3 mL of the 7:3 mixture of 10 mmol NaOH (aq) and dimethylsulfoxide (DMSO) was added to the NPs solution. The solution is stirred for 15-30 minutes, and tetrahydrofuran (THF) is added to get precipitation of NPs. NPs are collected using centrifuge, and washed several times using THF. After the multiple centrifugation, the NPs are fully dried in the desiccator for overnight to several days and made into dry powder for the further step.
Various concentrations of NPs solutions are used to test the gelation. All concentrations are made using the same ratio between NPs to Ca 2
+ ions. For example, 900 mg/mL sample of
NPs gel is made by using following two steps. First, 900 mg of NPs are dissolved in 735 [tL of
10 mM NaOH (aq). We made sure all NPs are dissolved in NaOH (aq) solution using brief sonication if needed. Then, 265 VL of 0.1 M CaCl
2
-H
2
O(aq) solution is added to the NPs solution.
For the control samples, 265 [tL of water is added to the solution instead of CaCl
2
-H
2
0
(aq) solution. For other concentrations, the amount of CaCl
2
-H
2
O(aq) solution is calculated based on the weight of NPs. For every 17 mg of NPs, 5 RL of CaCl
2
-H
2
O(aq) solution is added. For the rest of the solvent to make a desired concentration, NaOH(aq) is used.
96

4.9.5. TEM of Gold Nanoparticles
TEM images were obtained using JEOL 200 instrument. The TEM grids were placed on the Kim-Wipe, and 1-2 drops of the dilute NPs solution (- 1-2 tL) were dropped on the grid and allowed it dry slowly. The images are analyzed using ImageJ software. The areas of NPs are measured, and the radius is calculated assuming the NPs are sphere.
4.9.6. The Rheology of Nanoparticle Gels
The rheology measurement is performed using Discovery Hybrid Rheometer (TA
Instruments). 4 mm parallel plates were used to load ~ 700 tL of the NP solutions/ or gels. First, the amplitude sweep has performed to determine the amplitude to be used for the frequency sweep. We chose 0.3% strain for all the frequency sweeps that we performed. Then, the frequency sweep has performed at the constant temperature of 10 'C and the constant strain of
0.3%. When the frequency sweep is done, the temperature sweep is performed from 10 'C to
70 'C under the constant frequency of 10 rad/s.
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