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Development, synthesis and characterization of novel
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presented by
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DEVELOPMENT, SYNTHESIS AND CHARACTERIZATION OF NOVEL
MULTIFUNCTIONAL POLYMER-SECURED SELF-REGISTERING MOLECULAR
ARRAY SYSTEMS

By

Hanmi Xi

A THESIS

Submitted to
Michigan State University
in partial fiJlfillment of the requirements
for the degree of

MASTER OF SCIENCE
Department of Chemistry

2005

ABSTRACT

DEVELOPMENT, SYNTHESIS AND CHARACTERIZATION OF NOVEL
MULTIFUNCTIONAL POLYMER-SECURED SELF-REGISTERING
MOLECULAR ARRAY SYSTEMS

By
Hanmi Xi

The main objective of this dissertation is to develop novel multifunctional
polymer-secured self-registering molecular array systems with potential applications
in electronic equipments, magnetic, optical and biomimetic materials development.

In chapter 2, an idealized model with four functional elements was proposed. Based
on this model, three systems were developed and described (chapter 3 and 4). System
I was a biomimetic material with potential application as bone filler or scaffold of
bone tissue engineering. System 11 and III can be used as templates for extensively
ordered nanoparticle structure fabrication. System 111 also has polydiacetylene
function in the electronically and/or optically responsive region, which enables the
system to respond to different stimulae, such as temperature, UV light, etc. The
synthesis for these systems is very simple and highly efficient. The systems are stable
because of the polymerization, and at the same time, very sensitive to the
environmental factors that impact on the overall organization. In chapter 5, the three

self-registering polymeric systems were evaluated with a variety of methods.

ACKNOWLEDGEMENTS

This thesis is the result of three years of work whereby I have been accompanied and
supported by many people. I am grateful to now have the opportunity to express my
gratitude to all of them.

The first person I would like to thank is my advisor Professor Rawle I. Hollingsworth.
During these years I have known Dr. Hollingsworth as a sympathetic and very
knowledgeable person. His enthusiasm in probing the scientific truth, and inspiring
remarks on science and philosophy have made a deep impression on me. I owe him lots
of gratitude for showing me a brand new way to look at life and science. Besides being an
excellent supervisor, Dr Holingworth was also a good friend to me. I am really glad that I
have come to know Dr. Hollingsworth. I would also like to thank people who work in Dr.
Hollingworth’s group who gave me the feeling of being at home at work, especially Dr.
Carol Mindock who proof-read my thesis while busy teaching.

I feel a deep sense of gratitude to my parents who provide me the great support and
inspiration for my journey through life. Also many thanks to my dear friends, without

your help and support I would not be able to achieve anything.

iii

TABLE OF CONTENTS

LIST OF TABLES ................................................................................................. vii
LIST OF FIGURES .............................................................................................. viii
CHAPTER 1

THE USE OF ORGANIC TEMPLATES FOR THE STRUCGTURED AND
CONTROLLED ORGANIZATION OF METALS, METAL OXIDES AND

SALTS ................................................................................................... 1
Abstract .............................................................................................. l
1.1 Introduction ................................................................................. 2
1.2 Biomineralization as a model for templated organization of metal oxides and
salts ................................................................................................... 2
1.2.1 Proteins as biomineralization templates ................................................ 3
1.2.2 Polysaccharides as biomineralization templates ..................................... 6
1.2.3 Lipids as biomineralization templates .................................................. 7
1.3 A view of synthetic templated mineralization systems ............................... 10
1.4 Biomineralization mimetic systems using synthetic organic molecular or
biomolecular assemblies as templates ......................................................... 14
1.4.1 Surfactant molecular assemblies as templates .......................................... 16
1.4.2 Synthetic polymers as templates ......................................................... 18
1.4.3 In-vitro biological molecules as templates ............................................. 22
Bibliography ....................................................................................... 26

CHAPTER 2

A MULTIFUNCTIONAL POLYMER-SECURED SELF- REGISTERING

MOLECULAR ARRAY SYSTEM .............................................................. 33

Abstract ......................................................................................... 33
2.1 A Polymer-secured self-registered molecular arrays system ......................... 34
2.2 Multifunctional system .................................................................... 38
2.2.1 Polar region modification ............................................................... 39
2.2.2 Non-polar region modification ......................................................... 42
2.2.3 electronically and/or optically responsive region .................................... 43
Bibliography ..................................................................................... 48

CHAPTER 3

DESIGN OF THREE POLYMER-SECURED SELF-REGISTERING MOLECULAR

ARRAYS SYSTEMS ............................................................................... 49

iv

Abstract ........................................................................................... 49
3.1 Polymer-secured self-registering molecular arrays with phosphoric acid group in
the polar region and saturated hydrocarbon chains (system I) ............................ 50
3.2 Polymer-secured self-registering molecular arrays with thio-ether groups in the
polar region and saturated hydrocarbon chains in the nonpolar region (system 11). . .53
3.3 Polymer-secured self-registering molecular arrays with thio-ether groups in the
polar region and diacetylene moieties in the nonpolar region (system 111). . . . . ....54

3.3.1 Polar region of system III ............................................................... 54

3.3.2 Hydrophobic alkyl chain spacer region of system 111 ................................ 59

3.3.3 Electronic and/or optically responsive region of system 111 ........................ 59

Bibliography ..................................................................................... 63
CHAPTER 4

PREPARATION AND CHARACTERIZATION OF THE THREE POLYMER-
SECURED SELF-REGISTERING MOLECULAR ARRAY SYSTEMS..........64
Abstract ........................................................................................... 64
4.1 General strategies for polymer-secured self-registering molecular array system
preparation ....................................................................................... 65
4.2 Preparation of a stabilized phospholipid biomembrane monolayer mimetics
(system I) ........................................................................................ 67
4.3 Preparation of nanoparticle arrays on self-standing and unsupported
two-dimensional self-registering polymeric systems (system 11 and III). ...............72
4.3.1 A two dimensional self-registering polymeric system without an electronically

and optically responsive region (system 11) .................................................. 73
4.3.1.1 An unsupported membrane made from polymer II ................................ 76
4.3.1.2 A freestanding film made from the sulfur functionalized self— registering
polymer II ........................................................................................ 78
4.3.2 A two dimensional self-registering polymeric system with electronically and
optically responsive region (system 111) ..................................................... 80
4.3.2.1 Preparation of freestanding films made from the diacetylene incorporated
self-registering polymers ....................................................................... 84
4.3.2.2 Preparation of unsupported film made from the diacetylene incorporated
self-registering polymer ........................................................................ 85
4.4 Experimental Section ....................................................................... 86
Bibliography ..................................................................................... 93

CHAPTER 5

EVALUATION AND CHARACTERIZATION OF THE SYSTEMS ...................... 94
Abstract ......................................................................................... 94

5.1 Evaluation of surface coverage of a polyethylene substrate by polymer I and
characterization of the modified surface after calcium phosphate deposition ........96
5.2 Fluorescence and morphology study of unsupported membrane prepared with
self— registering polymer II and the observation of rod shape CdSe single

crystals .......................................................................................... 98
5.3 Fluorescence study of a self-standing edge-supported membrane prepared with
the self-registering polymer II ............................................................. 104
5.4 Optical property and High long range order of system 111 ........................... 107

5.5 Microscopy study of self standing membrane prepared with polymer III. . .. l 10
5.6 Scanning Electronic Microscope (SEM), Atomic Force Microscopy (AFM) and
fluorescence characterization of unsupported polymer III membrane and the

membranes incorporated with CdS or ZnS ................................................. 112
5.7 Conclusion ................................................................................. 125
5.8 Experimental Section ..................................................................... 127
Bibliography .................................................................................... 1 30

vi

LIST OF TABLES

Table 1.1 Use of organic templates to mediate and control the formation of inorganic
systems ................................................................................................ 15

vii

LIST OF FIGURES

Figure 1.1 Schematic illustration of mesophases formed by lipid self assembly ............ 9
Figure 2.1 Overview of the multifunctionalized polymer-secured self-registering
molecular array system .......................... I ................................................... 39
Figure 2.2 Electronically and/or optically responsive groups ................................. 44
Figure 2.3 Biphenyl conformation ................................................................ 45
Figure 3.1 Table illustration of system 1 ......................................................... 51
Figure 3.2 Polymer-secured self-registering molecular arrays with phosphoric acid groups
in the polar region ................................................................................... 52
Figure 3.3 Table illustration of system 11 ......................................................... 53

Figure 3.4 Self-registering polymeric system with sulfur functions in the polar region...56

Figure 3.5 Table illustration of system 111 ........................................................ 57
Figure 3.6 The polymerization of diacetylene ................................................... 61
Figure 3.7 Polymer-secured self-registering molecular arrays with thio-ether group in the
polar region and diacetylene moieties in the nonpolar region ................................. 62
Scheme 4.1 General strategy 1 .................................................................... 65
Scheme 4.2 General strategy 2 .................................................................... 66
Scheme 4.3 Mechanism of Michael addition of an aminothiol to the furanone ............ 67

Figure 4.1 Comparison of a natural phosphatidic acid self-assembled system and the
stabilized phospholipid biomembrane monolayer mimetic I .................................. 69

Scheme 4.4 Synthesis of system I ................................................................. 70

Figure 4.2 Illustration of the phosphatidic acid mimetics’ template control over
calcification .......................................................................................... 72

viii

Figure 4.3 Polymer-secured self-registering molecular arrays with thio-ether group in the

polar region (system 11) ............................................................................ 74
Figure 4.4 Ordering effects of the two dimensional template on nanoparticles ............ 75
Scheme 4.5 Synthesis of sulfur functionalized self-registering polymer II .................. 76
Figure 4.5 Preparation of an unsupported membrane made from polymer 11 ............... 78
Figure 4.6 Preparation of freestanding membranes made from 11 ............................ 79
Figure 4.7 Diacetylene functionality incorporated self-registering polymer III ............ 82

Scheme 4.6 Synthesis of a 20C-self-registering polymer incorporating diacetylene
(polymer III) .......................................................................................... 82

Scheme 4.7 Synthesis of a 30C-se1f-registering polymer incorporating diacetylene
(polymer IV) ......................................................................................... 83

Figure 4.8 Secondary polymerization of organic template formed with diacetylene
incorporated self-registering polymer ............................................................. 84

Figure 5.1 Structure of cationic dye crystal violet ............................................. 96
Figure 5.2 Comparison of bare polyethylene piece and polyethylene piece modified with
polymer I after crystal violet treatment .......................................................... 97

Figure 5.3 Dark field optics micrographs of polymer modified polyethylene
calcification ......................................................................................... 98

Figure 5.4 Comparison between a polymer I modified surface and a bare ethylene
surface ............................................................................................... 98

Figure 5.5 Fluorescence micrograph of cadmium selenide modified unsupported
membrane made from polymer II ............................................................... 99

Figure 5.6 Template growth and release of Au-CdSe-Au nanowires and SEM images of

Au—CdSe-Au nanowires .......................................................................... 101
Figure 5.7 Single crystals of cadmium selenide under dark field optics ................... 103
Figure 5.8 A dark field micrograph of CdSe single crystal ................................... 103

ix

Figure 5.9 Phase contrast micrograph of smaller sized rods dispersed on porous
membrane .......................................................................................... 104

Figure 5.10 Fluorescence micrograph of cadmium selenide modified pinhole supported
membrane made from polymer II. ............................................................... 105
Figure 5.11 Fluorescence micrograph of a free-standing membrane prepared with

polymer II ............................................................................................ 106

Figure 5.12 Fluorescence micrograph of self standing membrane prepared with polymer
II modified with CdS and ZnS ................................................................... 106

Figure 5.13 The membrane prepared from Polymer III becomes blue after UV

excitation ............................................................................................ 108
Figure 5.14 Chromatic alteration of polydiacetylene after UV irradiation ................. 108
Figure 5.15 X-ray spectra for system 111 ........................................................ 109

Figure 5.16 Curling structure of self standing membrane prepared with polymer
III ..................................................................................................... 110

Figure 5.17 Micrographs of self-standing membranes prepared from unmodified polymer
III membrane ....................................................................................... 111

Figure 5.18 A photo gallery of fluorescence micrographs for lamellar self standing
polymer III membranes and polymer III membranes modified with ZnSe, CdSe, ZnS and
CdS .................................................................................................. 114

Figure 5.19 Maltese cross pattern under polarization microscope .......................... 116
Figure 5.20 Comparison of unsupported membrane prepared with polymer III and the
ones that incorporate with CdS and ZnS ....................................................... 116

Figure 5.21 A photo gallery of fluorescence micrographs for lamellar self-standing
polymer III membranes ............................................................................ 117

Figure 5.22 Scanning electronic microscope image of unsupported polymer III
membrane ........................................................................................... 119

Figure 5.23 SEM micrograph of CdS (A) and ZnS (B) treated polymer III membrane with
cracking patterns ................................................................................... 120

Figure 5.24 higher magnification of SEM micrograph of CdS (A) and ZnS (B) treated
polymer III membrane showing no sign of large crystals .................................... 121

Figure 5.25 The flat, smooth thin surface of the membrane treated with CdS on a clean
silicon wafer surface at 300 nm .................................................................. 122
Figure 5.26 Different distributions of nanoparticels ........................................... 123

Figure 5.27 Distorted columnar structure of polymer membrane observed at size 100 nm
shown in three dimensional and two dimensional ............................................. 125

xi

CHAPTER 1

THE USE OF ORGANIC TEMPLATES FOR THE STRUCTURED AND
CONTROLLED ORGANIZATION OF METALS, METAL OXIDES AND SALTS

ABSTRACT

Biomineralization is the facilitated deposition of inorganic substances in organic
matrices. Proteins, polysaccharides and lipids frameworks provide the templates for
regulated aggregation or nucleation of inorganic minerals in the biological system. This
process is used as the guiding principle in the development of new materials.
Surfactants, copolymers, dendrimers and in-vitro biological molecules are fabricated
into membranes or templates to control nucleation and growth of mineral phases. Salts,
metals and metal oxides are incorporated into the organic-inorganic composite system.
In this process, the structures of the organic-inorganic composites are highly controlled
from atomic to macroscopic levels, resulting in complex architectures that provide
multifunctional properties. The composites have found applications in many areas such

as electronics, solar cell, magnetic recording, etc.

 

 

1.1 Introduction

The goal of this project is to control the deposition of arrays of inorganic materials
such as salts, metals and metal oxides on organic templates to form ordered materials.
The materials should have extended arrays, periodic structures or very defined local
arrays of limited scale in one dimension such as nanosystems. Strategies to accomplish
this require the development of advanced organic chemistry based technology and

materials to provide the underlying templates to facilitate array formation.

1.2 Biomineralization as a model for templated organization of metal oxides and
salts.

The prototypical systems in which soft organic molecules are used as templates to
facilitate the arraying of inorganic materials are biological in origin. Examples of these
include bone and teeth formation. This entire area is covered under the general theme of
mineralization.

Biomineralization is the study of the formation, structure and properties of inorganic
solids deposited in biological systemsl. It is widely seen in nature. Proteins, lipid
structures and larger macromolecular frameworks provide the templates to direct the
aggregation or nucleation of inorganic minerals. The modulation of the organic
components can work through interface interaction or through forming supramolecular

organic-inorganic composites. Various interactions such as chemical bonding,

electrostatic attraction or repulsion, mechanical stress, or spatial confinement define the
properties of the systemz. The structures of biocomposites are highly controlled from
atomic to macroscopic levels, resulting in complex architectures that provide
multifunctional properties.

Inorganic minerals in nature include ion oxides in magnetobacteria, magnesium
silicates, calcium carbonates, and calcium phosphates in invertebrate shells and vertebrate
skeletons. The most common organic components in organisms include lipids, proteins
and polysaccharidesz. Lipids are water insoluble cellular molecules. These amphiphilic
molecules are composed of one or more hydrocarbon (hydrophobic) tails and a polar
organic (hydrophilic) head group. The most abundant macromolecules are the proteins.
Proteins are polymers of amino acids joined covalently through peptide bonds. Examples
of using proteins as templates for biomineralization include collagen as a major
component in bones, S-layers in bacteria, and an organic matrix in mollusks or diatom
cell walls. The final commonly seen biomolecules used as templates are polysaccharides.
Polysaccharides contain more than 20 connected monosaccharides. Important
polysaccharides found in nature include cellulose in plants and chitin which is the second

most common structural fiber in animals.

1.2.1 Proteins as biomineralization templates:

The organic matrix in situ can control both the morphology and the orientation of

the crystal growth. The matrix in mollusks, for example, is composed of two parts,
EDTA (ethylene diamine tetraacetic acid) soluble part and insoluble part. The EDTA
soluble part is mainly composed of hydrophilic anionic proteins with functional groups to
nucleate the desired mineral and/or interact with the growing crystal faces3. The insoluble
part is composed of hydrophobic proteins and polysaccharides. It is a scaffold that helps
to array the functionalized hydrophilic part. These components work together to form the
complex structures found in mollusks.

Similar results were found in the cell walls of diatoms. Diatoms are unicellular algae
with cell walls predominantly composed of a biomineral derived from hydrated silica
($02), in association with peptides and polyamines‘. The silica precipitation mechanism
is not very clear, but one hypothesis suggests that the organic matrix in silica deposition
vesicles promote and direct the process5 . Detailed investigations uncover the controlling
efiects of the organic matrix. Comparison experiments show that the organic matrix helps
the process of silica mineralization 6-- untreated supersaturated silicic acid solution,
which can remain stable for several hours, starts to precipitate right after the addition of
the organic component separated from diatom cell wall. This demonstrates the nucleate
assistant fimction of the organic component. In addition, the organic components can
control the size of the inorganic particles by their molecular weight. The addition of the
smallest organic component leads to formation of particles with diameters between

500—700 nm, while the mixture forms particles with the average diameters of < 50 nm.

Bone is one of the most important organic-inorganic biocomposites in higher
vertebrates. It is made of an organic matrix that is strengthened by the deposition of an
ordered special form of calcium phosphate minerals, hydroxy apatite (HAP). The organic
matrix is mainly composed of type I collagen fibers (95 %)7. HAP provides bone’s
hardness, and the organic collagen fibers provide flexibility. The tensile strength of the
fiber resists being stretched or torn.

The function of morphology and function of bones are closely connected with the
structure of the organic matrix. There are two forms of bones: cancellous (spongy) and
cortical (compact). Cancellous bones are made of a loosely packed and disordered porous
matrix, and only provide metabolic functions. Cortical bones, however, are composed of
organized and densely packed collagen fibers and minerals and provide structural support
and protection.

The general importance of proteins and peptides on nucleation and growing stages of
minerallization are continuously demonstrated by a variety of in-situ experiments on
crystallization of calcium carbonate. For example, chiral crystals of calcite were formed
in the presence of pure D-, L-Aspartic acid due to the preferential binding of the amino
acid enantiomer to the crystals which provide the best geometrical and chemical matchs.
Homo— or heteropolymers of amino acids also have significant effects on calcium

carbonate crystallizationg.

1.2.2 Polysaccharides as biomineralization templates

Chitin is a linear polysaccharide composed of
B—(l,4)2-acetamido—2-deoxy-D-glucopyranose residues. It is produced by a variety of
animals, insects and fungi"). Chitin can be found in one of the three crystalline forms”:
a-chitin, B—chitin and Y -chitin.

a-Chitin forms the organic matrix of the shell of pink shrimp Pandalus borealis.
Calcium carbonate associated with the chitin is amorphous, while after continued frozen
storage, the amorphous phase crystallizes into two forms—calcite and vaterite. The chitin
is an integral part of the crystallized regions and thought to be important in calcium
carbonate deposition”.

The organic matrix found in the internal shell of the cuttlefish (Sepia oflicinalis) is a
B—chitin template. Sheets of aragonitic calcium carbonate formed on the template were
separated by pillars in a manner similar to a honeycomb”. The pillars are sigrnoidal in
cross section, which provides maximum resistance to crushing with minimum mass of
material. B-Chitins also serve as an organic scaffold in the shell of Lingula unguis for the
apatite crystals to precipitate on. The larnella structure has improved mechanical
properties. The fiber axis of B-chitin being parallel to the c axis of apatite and the close
relationship of unit cell dimensions of chitin and apatite show the orientation effect of
B—chitin”.

Another study determined the modification effect of polysaccharides in the localization

of iron oxyhydroxide mineral precipitation in microorganisms”. The iron oxyhydroxide
mineral goes through a redox reaction to enhance metabolic energy generation. When
compared with representative K-edge X-ray absorption near edge structure (XANES)
spectra from various organic polymers, including proteins, lipids, and nucleic acids, the
spectrum of nonmineralized fibrils matches best with alginate, an acidic polysaccharide.
The author infers that the cells extrude the polysaccharide strands to localize FeOOH
precipitate outside the cell membrane to control the proton gradient for energy

generation.

1.2.3 Lipids as biomineralization templates:

One common biological strategy employed during the formation of biominerals is
controlling the location, size, composition, and shape of mineral crystals by the lipid
mesophases”. Mesophase materials have domain length scales of the order of a few
nanometers to a few hundred nanometers. In aqueous solution, due to the amphiphilic
property of lipids, they tend to self assemble into a variety of mesophase structures,
including micelles, reverse micelles, hexagonal and inverted hexagonal phases, bilayers
and bilayer vesicels (Figure 1.1). The form of the mesophase structures depends on many
factors such as the charge capacity of the headgroups, the length and degree of unsaturation
of the fatty acid chains, ambient temperature, pH, concentration and the presence of salts

and other solutes. A common and biologically important mesophase structure is the

lamellar bilayers. Most of the biological membranes have lipid bilayers as the major
building blocks. Together with membrane proteins and cholesterol, lipid bilayers fulfill the
functions including maintaining the cellular integrity, communicating with the environment
and other cells, storing compounds, cell fusion and metabolic pathways.

Phospholipids are lipids with a phosphate group at the polar end. Phospholipid
mediated biomineralization occurs in all different kinds of species and tissues. It controls
the nucleation kinetics”. The electrostatic, stereochemical and geometric interactions
between the phospholipids and the minerals orient the nucleation and growth of the
mineral. The phospholipids serve as a molecular blueprint for the conductive formation of
the inorganic phase. By changing the shape of the lipid matrix during crystal growth,
complex crystal shapes can be formed.

One example is the phospholipids mesophases controlling the formation of
coccoliths in the alga Emiliania Huxleyi's. Alga use coccoliths as an exoskeleton to
surround and protect itself. Coccoliths consist of 30-40 oval shaped calcite crystals.
During biomineralization, phospholipids self assemble into vesicles followed by the
infusion of calcium and carbonate ions. \Vrth the help of structural, chiral and chemical
complimentarity between the organic matrix and the inorganic minerals, oriented
nucleation of calcite crystals occurs. The vesicle compartment shape can be further
modified via active cellular cytoskeletal elements, and thereby refine the mineral crystal

shape. The phosphorlipid mesophase controlling leads to the highly ordered structures

found in coccoliths and other biominerals.

 

Figure 1.1 Schematic illustration of mesophases formed by lipid self assembly: (a)
micelle, (b) reverse micelle, (c) lamellar bilayer, (d) bilayer vesicle, (e) hexagonal, (f)
inverted hexagonal. (adapted from reference 16)

Magnetosomes are single crystals of magnetite (Fe304) or greigite (Fe3S4)
encapsulated in a bilayer vesicle. It is believed that the long chains of connected
magnetosomes with a permanent magnetic dipole that are often seen in the cytoplasm
of magetotactic bactia are used to navigate with respect to the earth’s magnetic field.
This form of magnetosomes shares many features with the coccoliths formation”.
Magnetosomes are formed in the vesicle compartment, and the formation is controlled

by the encapsulation and templating effects from the phospholipid membranes.

Molecular recognition at the inner face is likely to help control the magnetite crystal

orientation.

1.3 A view of synthetic templated mineralization systems

Table 1.1 lists recent research in using organic templates to mediate the formation
of inorganic layers.

The relevant research has been concentrating on two aspects: the organic template and
the inorganic materials that are organized by the template. The inorganic materials
include metal salts, metal oxides, metals and transition metals. The organic templates
studied include surfactants, polymers, and self-assemblies. These materials are useful in
magnetic, optical, electronic, pollution controlling and sensor development. All share the
use of an organic template to control and organize the aggregation and orientation of the
inorganic material and the organic-inorganic composite has the unique properties that
neither the organic nor the inorganic component can provide by itself.

Metal salts mainly include semiconductors and biomimetic materials. Semiconductors
such as cadmium selenide, are connected closely with nanotechnology and widely used in
electronics, optics and solar cells. With the help of the organic templates, the size of a
semiconductor can be brought down to the nanometer range. Properties of these
nanomaterials can be tuned by changing the particle size. For example, the band gap of CdSe

can be tuned from deep red (1.7 ev) to green (2.4 ev) by reducing the cluster diameter from

10

200 to 20 angstroms. Size tunable luminescence with efficiency on the order of 10 % was
observed for semiconductor nanocrystals at room temperature, which makes it a very good
candidate for solar cell materials. Nano-size materials also have unique properties in optics,
including non-linear optical properties. These have potential applications in optical
instruments, such as optical switches, and other components in all-optical devices. In
addition, the extremely small size makes it a good building block for nanoelectronic devices,
such as oneoelectron transistors or nanocapacitors.

Another branch of study is on biomimetics. Biomimetics is defined as microstructural
techniques that either mimic or are inspired by biological processes. It includes the study of
biological mechanisms and the development of biomimetic materials. The mechanisms of
formation of organic-inorganic biocomposites like bones in the human body are not very
well understood. Questions such as: ‘what controls the initial deposition of mineral crystals?’
and ‘whether a common mechanism of biomineralization exists?’ are still waiting to be
answered”. Advances in analyzing techniques continue to modify the answers to these
questions, and the information obtained can be applied in therapeutics for the treatment of
diseases in which mineralization is impaired or excessive, or in the development for artificial
tissues.

Metal oxides have many interesting applications in areas such as magnetic, electronics,
optoelectronics and solar cells. Ion oxides and titanium oxides are the most studied. Due to

the magnetic properties, ion oxides can be applied in security paper, color imaging,

information storage, bioprocessing, magnetic refrigeration, ferrofluids and
magnetosomes used in gene therapy. In one of the studies, ferrofluids of iron nanocrystals
were processed by thermal decomposition of iron carbonyl in a mixture of decalin with
oleic acid as the surfactant. After exposure to air for about one month, a hexagonal close
packing of hematite (0 -Fe203) was formed. Block c0polymer films were also applied as
templates”. (NORCOOH]30[MTD]3oo (NORCOOH = 2-norbomene-5,6-dicarboxylic acid;
MTD = methyltetracyclododecene) were synthesized into free-standing nano-composite
films, and the iron oxide nanoclusters (Y -Fe203) were formed in the microdomain of the
copolymer.

Titanium oxide, on the other hand, is transparent, which makes it very useful in
optoelectronics and solar cell developmentn. In photovoltaic films in solar cell application,
except for templating, poly-2-methoxy-5(2-ethyl)hexoxy-phenylenevinylene (MEH-PPV)
was used as both the active photgeneration medium and the hole transporter.

Silica, metals and transitional metals are also common inorganic materials studied in
inorganic-organic composites. Silica is mainly used in preparation of meso/microporous
materials. In the past 60 years, there has been substantial interest in the synthesis of
meso/microporous materials in view of their properties and their applications in catalysis,
absorption and templates for further modification and reaction”. Amines and related
compounds (quaternary ammonium cations), linear or cyclic ethers, and coordination

compounds (organometallic complexes) have been the most commonly used organic

12

templates for microporous material preparation. In most cases, an adequate matching
exists between the geometries of the organic species and those of the microporous cage or
channel network. Therefore, the molecules have template effects, around which the mineral
framework is built.

The dimensions and accessibility of pores in the microporous structured hybrid
materials (with pore diameter below 2 nm) were restrained to the sub-nanometer scale.
This limited its application to small molecules. In order to overcome the limitation,
molecular sieves with larger pore size assemblies (micellar aggregates rather than
molecular species) were used as templates to make mesoporous silica and aluminosilicate
compounds (with pore diameters between 2 nm to 50 nm). The M418 family developed
by a research group at Mobil Oil is the most widely studied“.

Metals can be prepared into different shapes and morphologies by using different
templates. By the addition of a colloidal or dispersed solid to an about-to-gel silica
solution, followed by supercritically drying, can form composite aerogels in which
nanoscopic surface and bulk properties of each component are retained in the solid
composite. The volume fraction of the second solid can be varied above or below a
percolation threshold to tune the transport properties and thereby design nanoscale
materials for chemical, electronic, and optical applications. The composite aerogel is
widely used as the template for making three- dimensional metal nanostructures.

Metal nanowires have potential applications in microelectronic devices, magnetic

records et. al. DNA templates, mesoporous silica matrices”, and copolymers such as
polystyrene-polymethylmethacrylate (PS-PMMA) have been used to make metal
nanowires. Another commonly seen structure is a nanotube such as a peptide
functionalized nanotube. Surfactants have been reported as the templates for making

nanotubes.

1.4 Biomineralization mimetic systems using synthetic organic molecular or
biomolecular assemblies as templates

The underlying molecular mechanism of biomineralization has lead to the idea of using
organic compounds as templates to generate ordered inorganic structures and materials.
Inorganic materials have all different morphologies and crystal structures, but the
varieties are very limited compared to the structures of superrnolecules and other organic
compounds. Both natural bioorganic and synthetic organic templates have been applied in
these systems”. Transferring the structures from organic templates to the inorganic
minerals allows molding of the inorganic structures on a microscopic level. This method

provides a new way to form materials with complex inorganic structures.

Table 1.1 Use of organic templates to mediate and control the formation of inorganic

 

systems
Materials Application Template Details Ref
CdSe:Si, one electron transistor, microelectronic
metal salts Electronics Inorganic architecture [72,73]
cP Nanocapacitors and nanotransistors [74]
S CdS:polyethylene oxide; SnSzz CTAB [75,76]
Biomineralization P CaCO3, Calcium phosphate, repairing bones or teeth [77,78]
Opitcs P CdSe:PPV,LED, optical switch
P CdSe, CdS [79]
Solar Cell P CdSezMEH-PPV; photovoltaic films used in photo- [80,81]

metal oxides Magnetic

metals

Transition
Metal

electronics

Optoelectronics

Solar Cell
Enviromental
Engineering

catalyst/ molecular

sieves

Optical

electronics

Magnetic

membrane

separation

SCIISOI'

cP

"U'U‘UUDUJ

U)

SA
DNA
cP

S

Voltaic and photosynthetic systems
Fe203: [NORCOOH]3[MTD]300, security paper,color [82-89]
imaging, information storage,bioprocessing,magnetic

refrigeration, ferrofluids ,megnetosomes used in

gene therapy

F6203, Triol surfactant [90]
SnOz mesoporous structure [91]
TiOz, waveguide [92]
TiOz : MEH-PPV [93]
heavy metal: Mercaptopropylsilane [94]
Tizsilica; tubule Pd: M418; hollow tubules, Al [95-97]
Auzsilica aerogel [97]
Ptzpeptide functionized nanotube; Cu [98,99]
Ag nanowire [100]
Co or Cu: PS-PMMA, magnetic nanowire array [101]

as ultrahigh-density Recording media

mesoporous Si, biomolecular [102]
Si as catalyst and molecular sieves
[Re(CO)3(L)]4(L-pyradine, 4,4’-bipiridine)] for VOC"
sensor [103]

 

VOC: volatile organic compound

P: polymer
S: surfactant

SA: self-assembly

CP: copolyrner

Generally, synthesis of these materials includes two steps27. First the organic template
with preformed functional groups or self-assembled entities is brought into contact with
the inorganic precursor or small particles of the inorganic minerals. This procedure will
form organic-inorganic hybrid materials with the inorganic material deposited on the
surface of the organic templates. The following step involves removing the organic
template. This step is optional. The removing methods include heat treatment”,
microwave irradiation29 or washing with organic solvents”. After the organic template is
removed, the isolated inorganic material with a morphology directly related with its
organic template is formed. It can be further modified with other organic compounds to
be functionalized. If the organic template is not removed, the organic-inorganic hybrid

material has proved to have many interesting applications as well.

1.4.1 Surfactant molecular assemblies as templates:

The idea of synthesizing mesoporous solids in the presence of surfactant solutions was
proposed by Beck et al. in 19923 '. In the light of the rich variety of elaborate inorganic
structures derived from closed-packed arrays of large vesicles in nature, Walsh and Mann
have synthesized networks of porous calcium carbonate from microemulsions of
didodecyldimethyl ammonium bromide (DDAB), tetradecane and aqueous calcium
bicarbonate solutions. Adding polystyrene spheres in the mixture gives rise to structures

that resemble the coccospheres of certain marine algae”. Singh and Kosuge form hollow

spherical silica with alkaline hydrolysis of tetraethylorthosilicate(TEOS), octylamine
and aqueous HCl. The Coulombic interactions between the protonated amines in acidic
condition result in the spherical shape of the octylamine aggregates”. Gemini-type
surfactants such as RNH(CH2)2NH2 or NH2(CH2),.NH2 were used to form highly stable
vesicular porous silica sturcures with very thin, robust shells with the potential
application as molecular sieves”.

Unilamellar vesicles have recently been reported as templates to transport morphology
into silica by German and coworkers”. Through controlling the rate of acidification of
a solution of sodium silicate, butanol and myristyltrimetrylammonium bromide
(C14TMAB), pillar-like structures containing silica spheres were obtained”. Spherical
aluminum phosphate was made with vesicle templates of alkylammonium dihydrogen
phosphate aggregates when pseudoboehnrite was added. The procedure shows strong
similarity to the formation of rediolaria and diatom skeletons”.

With the exception of spherical structures, surfactant systems can form tubular and
wirelike structures as well. Baral and Schoen coated phospholipid tubules with silica by
sol-gel methods38 to form tubule shaped silica. Single silica nanotubes, as well as bundles
of tubes, have also been prepared by using the sol-gel method with TEOS and a template
based on laurylarnine hydrochloride”. In a similar experiment, magnetic and

nonmagnetic iron oxide tube-like structures were formed by using mixtures of a

sugar-based lipid and its anionic sulfated derivative as templates“). The mineralization

starts at the anionic sites followed by the formation of organic-inorganic composites.
However, dense and uniform coatings cannot be formed.

It has been reported that BaCO3 nanowires were prepared through reaction of barium
and carbonate ions in the cores of nonionic reverse micelles“. The preferential
orientation of surfactant molecules along the surface planes parallel to one crystal axis

after nucleation is assumed to lead the wirelike crystal growth.

1.4.2 Synthetic polymers as templates

Many synthetic polymers have been used as templates, most of them are
polystyrene-based. The inorganic minerals and preparation procedures vary. Hollow
ZnS shells were prepared through sonicating carboxy-modified polystyrene particles with
zinc acetate and thioacetarnide, followed by calcinating the polymer”. This method is
also extended to other materials such as PbS and different morphologies of TiOz. Another
preparation method use miniemulsions which consist of small oil or water droplets

303% nm in diameter“.

The miniemulsions containing styrene or other easily
polymerizable monomers are used as templates. After polymerization, the latexes are
covered with clay sheets, which react then with tetramethylorthosilane through a
polycondensation reaction to form nanocapsules with thicknesses of about 1.5 nm.

Other polymer templates explored include poly(L-lactide) fibers“. Greiner and

coworkers processed the fiber by coating it with aluminum through physical vapor

deposition. Stiff single-walled aluminum tubes were formed afier thermal degradation
of the polymer templates. Poly(L-lysine)-amine aggregates were also used to form hollow
silica spheres by the sol-gel transcription method”. lts successful transcription into silica
depends on the hydrogen-bond donating ability of the amine.

Cha et. al. tried to control the formation of ordered silica morphologies through
changing the degree of oxidation of the poly-L-cysteine domains of cysteine-lysine block
copolypeptide aggregates“. Under a nitrogen atmosphere, in its reduced form, spherical
silica was formed. When the reaction was performed in air in which the sulphydryl
groups of the copolypeptide were partially converted into disulfides, ordered columnar
silica was formed.

Amphiphilic block copolymers have been employed as templates. Compared with
surfactants, block copolymer templates are more stable against phase transition due to the
longer relaxation time.New developments in this field include growth of nanoparticles in
double-hydrophilic block copolymers". Double-hydrophilic block copolymers consisting
of a solvating poly(ethylene glycol) (PEG) block and a poly(ethylene imine) (PEI)
binding block were used to make monodisperse CdS nanoparticles with controllable size
and stability against oxidation for months due to its polymer shell.

Selectivity was achieved through preferential wetting of one of the copolymer blocks by
a metal in the system with the polystyrene-block-poly(methylmethacrylate)

(PS-b-PMMA) diblock copolymer as the template“. The parallel cylindrical domains of

PMMA with a repeat spacing of 50 nm was surrounded by PS. The selectivity of the
two copolymer domains is very apparent immediately after metal deposition for a wide
range of metals and small deposited amounts. The behavior of Au, Ag, In, Pb, Sn and Bi
was observed with Au and Ag prefening the PS domain, and In, Pb, Sn and Bi the
PMMA domain. In each of these cases, the preferred domain becomes the scaffold.

Methods based on polymer-assisted layer-by-layer assembly49 is applied to assemble
metal-based magnetic nanoparticles”. The idea is to exchange the stabilizers bound to the
particles with multifunctional polymers that attach to a substrate to control the assembly
of magnetic nanoparticles. The assembling process involves exchanging oleic acid/oleyl
amine around each magnetic nanoparticle with a functional polymer such as
poly(ethylenimine) (PEI). The PEI/FePt multilayer assembly that has been formed was
proven to be ferromagnetic. Such an assembly might be suitable for future
ultrahigh-density data storage media.

The exchanging method is also used to develop a general method to make a controlled
self-assembly of nanoparticles, which is also called the ‘ bricks and mortar’ approach5 1.
In this method, colloidal gold particles functionalized with recognition elements were
glued with polymers bearing complementary functionality.
Diaminotriazine-functionalized polystyrene was employed as the polymer template, and
gold particles were covered with an octanethiol self-assembled monolayer followed by

thiol placeexchange with a thymine-functionalized alkanethiol to form

20

thymine-functionalized colloids. Diaminotriazine-thymine three-point hydrogen
bonding interactions were used as the complement. The conformational flexibility of the
polymer compensates for irregularities in the size and shape of the aggregate structure,
allowing the efficient propagation of order during the self-assembly process.

Dendritic polymers represent an additional and unique well—defined polymer template
for nanoparticle growth52 and encapsulation”. Compared with other organic templates,
dendrimer structures have unique aspects in the chemistry of the terminal groups, the
generation-dependent size, the three—dimensional structure, the radial distribution
function of polymer density, and the presence of endoreceptors within the dendrimer.

Dendrimers were used to stabilize and control the growth of nanoparticles by
forming interdendrimer complexes. With this approach, Murphy and co-workers prepared
agglomerates of CdS and dendrimers“. Crooks and co-workers have prepared related
materials consisting of Au and dendrimers”, and Esurrri and co-workers have prepared Pt,
Au, and Ag nanoparticles stabilized by dendrimers absorbed to their exterior“. New
results indicate that even semiconductor quantum dots (CdS) are accessible by this
synthetic route”, and Amis et. al. have further expanded the scope of
dendrimer-encapsulated nanoparticles by preparing them within monolithic

dendrimer-containing polymer composites”.

21

1.4.3 ln-vitro biological molecules as templates

In-vitro biological systems use nature's biomaterials (macromolecules such as proteins,
glycoproteins, and polysaccharides) to control nucleation and growth of mineral phases and
thus manipulate the microstructure and physical properties of the system.

In general, biomolecules can self-assemble into a wide range of structures. Proteins
in particular can form intricate structures that can be readily manipulated and
firnctionalized because their synthesis is genetically directed. Natural materials such as
bacterial surface layer proteins”, virus capsidsf’0 and phage“ have also been used as
templates and in other nanoscale applications.

A study on cyanobacterial S layer in 1992 demonstrated that a crystalline surface layer
can function as a template for fine grain mineralization”. Later studies show that surface
protein monolayers on substrates and sheet-like self-assembly products in suspension
promote the precipitation of monodisperse cadmium sulfide63 and gold nanoparticles“.

Virus protein cages (capsids) exist as well defined large individual molecules instead
of aggregate ensembles, such as lipid vesicles, and thus can be used as templates for
nano-engineering of materials with precise and controllable morphologies. A range of
polyoxometalate species such as vanadate, molybdate and tungstate spatial-selectively
crystallize within the cowpea chlorotic mottle virus capsid. The same mineralization

reaction with tungstate is also possible with Norwalk Virus”. Douglas et. a1. believed that

22

the crystallization is electrostatically induced at the basic interior surface of the protein.
The negatively charged polyoxometalate ions (such as H2WIZO42'O’) aggregate at this
interface facilitating crystal nucleation. Thus, the author suggested that the protein shell
acts both as a nucleation catalyst and a size and shape constrained reaction vessel.

Monodisperse biomaterials with anisotropic shape are promising components as
well—ordered structures. A recent study involves genetically engineered M13
bacteriophage, viruses with monodisperse size and shape, and ZnS precursor solutions to
self—assemble highly oriented, self-supporting films“. In this system, both the length of
bacteriophage and the type of inorganic materials can be easily modulated through
genetic modification and selection. The resulting quantmn dot (QD) hybrid film material
was ordered at the nanoscale and at the micrometer scale into 72-um domains. These
domains repeated continuously over a centimeter length scale. Moreover, viral
suspensions containing ZnS QDs were prepared in which the liquid crystalline phase
behaviors of the hybrid material was controlled by solvent concentration and by the use
of an applied magnetic field.

Polysaccharide dextran is used to prepare self-supporting macroporous frameworks
of silver, gold and copper oxide. as well as composites of silver/copper oxide or
silver/titanium“. Magnetic sponges can be similarly prepared by replacing the metal salt
precursor with preformed magnetite nanoparticles. The use of dextran as a soft

sacrificial template for the fabrication of metal and metal oxide sponges should have

23

significant benefits over existing technologies because the method is easy, inexpensive,
environmentally benign, and amenable to scale-up and processing.

Lamellar DNA-cationic membrane complexes, originally studied for gene therapy, are
a new class of hierarchically organized nanoporous biopolymer- membrane system. DNA
has a diameter of only 2 nm and micrometer-long distribution of well-defined sequenced
DNA bases.

Braun and coworkers made a DNA-template-based silver nanowire around 100 nm
thick and 15 nm long“. The metallization was performed in three steps. First, the DNA
was fixed between two gold electrodes. The sodium was then exchanged with a solution
containing silver ions, which bind to the DNA bases. In the third step the DNA was
metallized with silver using a standard photographic enhancement technique. The
nanowires made this way showed a resistance of several mega-ohms as a result of the
coagulation of silver, which leads to clusters with a diameter of 50 nm. Pladium clusters
with a diameter range from 3 nm to 20 nm controlled by different length of reduction
time was reported in another work by Schackert and coworkers”.

In recent research, the synthetic biomolecular templates consist of periodic structures
with parallel anionic DNA chains confined between stacked two-dimensional cationic
lipid sheets. DNA and cationic lipids self-assemble into multilamellar nanoporous
complexes in which a one dimensional DNA lattice is intercalated between lipid

membranes. Divalent counter ions can be organized into the inter-helical DNA pores, and

24

the resultant arrays of ionic precursors can be used to grow inorganic crystals. CdS was
used in a model system. Cd2+ ions were organized within the interhelical pores between
DNA strands. Subsequent H28 treatment forms CdS nanorods with controllable widths
and crystallographic orientation70 aligned parallel to the negatively charged
sugar-phosphate DNA backbone due to the strong electrostatic interactions. A similar

sandwich structure is reported elsewhere with tubule networks".

25

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95 Ymg, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem, Int. Ed. 1999, 38, 56.
96 HP. Lin, C.Y. Mou, Science 1996, 273, 765-768.

97 Morris, C. A.; Anderson, M. L.; Stroud, R. M.; Merzbacher, C. 1.; Rolison, D. R.
Science 1999, 284, 622.

98 Yu, Lingtao; Banerjee, Ipsita A.; Matsui, Hiroshi., Journal of Materials Chemistry,

31

2004, 14(4), 739-743.

99 Banerjee, Ipsita A.; Yu, Lingtao; Matsui, Hiroshi., Proceedings of the National
Academy of Sciences of the United States of America, 2003, 100(25), 14678-14682.

100 Eichen, Y.; Braun, E.; Sivan, U.; Ben-Yoseph, G, Acta Polymerica,1998, 49(10-11),
663-670

101 T. Thum-Albrecht, J. Schotter, G A. Kastle, N. Emley, T. Shibauchi, L. Krusin-
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102 Nagamine, Shinsuke; Kurumada, Ken-ichi; Tanigaki, Masataka; Endo, Akira.
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103 Keefe, M. H.; Slone, R. V.; Hupp, J. T. Czaplewski, K. F.; Snurr, R. 0.; Stem, C. L.
Langmuir 2000, l6(8),3964

32

CHAPTER 2

A MULTIFUNCTIONAL POLYMER-SECURED SELF- REGISTERING
MOLECULAR ARRAY SYSTEM

ABSTRACT

An idealized model which combines the idea of self assembly and polymer architecture is
proposed. The template is composed of polymer-secured self-registering molecular arrays.
It is robust due to the polymerization, and at the same time, very sensitive to the
environmental factors that impact on the overall organization. The system has four major
elements: the polar recognition and chemo-responsive region, the hydr0phobic alkyl
chain spacer region, the electronically and/or optically responsive region and
functionalized anchor region for attachment to the substrates. All the regions can be
functionalized or modified for different purpose. The polar region is hydrophilic, which
can incorporate chirality, polymerization moieties, biological recognition elements and
chemical functionalities for inorganic phases binding. The hydrophobic alkyl chain
region gives stability and order to structure, and separate different regions as a spacer.
The electronically and/or optically responsive region can have groups such as
polyphenylene, diacetylene, diphenyl or phenyl acetylene which response to different

electronical or optical signals.

33

2.1 A Polymer-secured self-registered molecular arrays system
There are two ways to make functional systems composed of many molecular units.
There are cases where these units are not covalently connected, but they are connected
through electrostatic forces, Van der waals interactions, 1: -stacking or hydrogen bonds.
This is referred to as self-assembly. In the other instance, the units are connected
covalently. These systems are referred to as polymers. Because of their architecture,
self-assembled systems are very sensitive to outside stimulae, such as pH and
temperature. Their properties are also very dependent on concentration, etc. On the other
hand, polymers tend to be more insensitive to these factors. Polymers have more
mechanical strength compared to self-assembled systems. They can be processed to make
stable rugged devices. The ideal system would have both the sensitivity of self-assembled
systems to environmental stimulae and the stability and ruggedness of polymers. The
design and fabrication of such systems is the objective of this work. To this end, we have
developed a lamellar chiral polymer-secured self-registering template capable of being
functionalized at a variety of levels. Such systems will be powerful materials for
producing functional materials.
This approach enables the possibility to construct hierarchically structured materials
with order at different scale lengths. There are two general approaches in making ordered

structures at different scale lengths—top-down and bottom-up approaches. The top-down

34

approach carves out or adds molecules to surfaces using such methods of soft
lithography or dip-pen lithography. The bottom-up approach, on the other hand,
assembles molecules into structures. Compared with the traditional top-down approach,
bottom-up fabrication provides numerous advantages, such as the possibility of
three-dimensional assembly, the likelihood of inexpensive mass fabrication and the
attainment of nanosized material]. For example, due to the resolution limit of optical
photolithography, it has to be replaced by e-bearn or x-ray lithographies for continued
miniaturization according to Moore’s Law. This exponentially raises the cost of
microfabrication below the 0.1 pm scale. The bottom-up approach does not have the
above limitation down to the 10 A scale, but at the same time it lacks high yields and
uniformity.

Self-assembly is one of the few practical strategies to arrive at the ordered structure
based on this bottom-up approach. The self-assembly process is defined as the
spontaneous organization of components into structurally well-defined aggregates joined
by noncovalent bondsz. The organization or the assembly forms the desired structures
through naturally intended phenomena, either through physical or chemical processes or
assisted by biomolecules to promote molecular selectivity and specificity. It also rejects
defects energetically, and therefore the degree of perfection is relatively highs. Molecules
and nanoclusters can be assembled thrOugh electrostatic and surface forces, hydrogen

bonds, hydrophobic and hydrophilic interactions“.

35

The strategy we used can be called polymer-secured self-registering molecular arrays
manufacturing. It borrows the self-arrangement idea from the self-assembling methods.
The amphiphilic molecules self register through hydrophobic and hydrophilic interactions.
Amphiphiles are ‘dual character’ molecules with two moieties at the ends having quite
opposite propertiess. One moiety is hydrophilic which is polar and ‘water loving’, while
the other end is hydrophobic which is nonpolar and ‘water hating’. The most well known
class of amphiphiles is surfactants, which are widely used in the detergent, cosmetic,
pharmaceutical and food industries. In addition, biological amphiphiles such as
phospholipids serve as building blocks of cell membranes.

The amphiphilic molecules can be regarded as building blocks for larger
superstructures such as structures with cubic or hexagonal symmetry or extended and flat
lamellar structures. In aqueous solutions, amphiphiles tend to self-register into aggregates,
with the hydrophilic moiety pointing outward to water and their hydrophobic moiety
pointing inward or interacting with other hydrocarbon groups. The solubilized aggregates
can be spherical or lamellar in shape. In our case, it has lamellar structure. The lamellar
structure has several advantages. It has biological analog models in living systems.
Therefore it is possible to make biomimetic systems to be used as sensors for signal
tranductions across a membrane, to study interactions between cells, or to detect living
cells and components. These systems can serve as models for cellular processes of

important biological events such as photosynthetic systems and vascular systems where

36

lamellar structures provide compartments. Due to its flatness, the lamellar structures
can be stacked to make a multilayer structure with long range order. The lamellar
structure can be easily applied to modify or functionalize surfaces. Since the reaction
sites are exposed in the lamellar structure, it is easy to characterize or modify the system.
For example, we can add polymerizable moieties in the head groups, or functionalized
groups in the nonpolar phase which bring high UV, fluorescence, nonlinear optical
property, semiconductor or conductor behaviors into the system.

The properties of the amphiphilic molecule and the higher level superstructures are
tightly linked. Extemal parameters such as temperature or concentration can also lead to
extensive rearrangements and structural changes on several scale lengths, in other words
the system is very sensitive to environmental stimuli. Elucidating the correlations
between molecular structures, external stimuli and macroscopic properties of these
materials is a major scientific challenge, and has potential applications in sensor
development.

Self-registering systems form well-ordered systems at the nanometer scale. Through
entropy minimalization, the whole process is spontaneous Therefore the process is
cost-effective, versatile and facile. However, self-registering systems have an inherent
shortcoming due to the non-covalent interactions. These systems are weakly connected
and not robust enough for practical applications. A good strategy is to combine

self-registering systems with the covalent polymerization systems.

37

Covalent polymerization is the most important strategy for preparing molecules with
high molecular weights". In covalent polymerization, a relatively simple, reactive low
molecular weight substance (a monomer) reacts with itself to produce a molecule
comprising many covalently connected monomers (a polymer). The molecular weight of
synthetic polymers can be high and easily prepared. However, the molecular structure is
simple and repetitive, and the polymerization process only provides limited opportunity
for controlled variation in the structure or for control of its three-dimensional shapez. For
example polymers donot usually form bilayer systems such as the system that we
developed. Also, polymers are not very sensitive to environmental stimuli. Therefore
their applications as sensors are limited, and the structures cannot be fine tuned by
altering conditions.

The combination of covalent polymerization and self-registering could make a system
with ordered structures through relatively simple synthesis with high sensitivity. In this
project, we are designing and synthesizing systems with the combined advantages of

self-assembly and covalently linked polymers, and yet avoid all the disadvantages.

2.2 Multifunctional system
As mentioned above, the properties of the amphiphilic molecules and the higher level
superstructures are tightly linked. The macroscopic properties of the material can be

modified through introducing functionalized elements to the amphiphilic molecules

38

 

Hydrophilic, chirality for optical activity, polymerizable
moieties, chemical functionality such as sulfur functional
groups binding metals to make conductive or magnetic,
biological recognition elements such as enzymes or its

substrates, DNA strand, antibody or antigen, fluorescent

Hydrophobic alkyl chain

groups

 

 

spacer region
Hydrophobic, to give stability and order to structure,

spacer to separate regions, insulator layer

 

 

 

 

Electronically and/or
optically responsive Groups such as polyphenylene, diacetylene, dlphenyl,
phenyl acetylene groups which can give properties of
I‘L‘UIOII fluorescence, conductance, chromism, liquid crystalline
: characters
Hydrophobic alkyl chain Hydrophobic, to give stability and order to structure,

spacer to separate regions, insulator layer

 

spacer region

 

 

l’unctiontllizcd flllCllOl‘ Interact with substrate or support, such as thiol group for gold
substrate, or silane for silica, alkyl chains for plastic
region for attachment to

 

 

 

substrate

 

Figure 2.1 Overview of the multifunctionalized polymer-secured self-registering
molecular array system

2.2.1 Polar region modification

Into the polar region, we can introduce chirality, polymerizable moieties, chemical
functionalities that are derivable, inorganic phases, biological recognition elements or
inorganic phases or fluorescent groups. Polar regions provide the site to secure the side
chains and control the space between the side chains at the same time. They also dictate

the packing pattern of the system. The polymerizable moieties of the polar region

chains and control the space between the side chains at the same time. They also dictate
the packing pattern of the system. The polymerizable moieties of the polar region
stabilize the whole system through covalent bonds. The chirality makes the system
optical active. When light passes through a polar layer with chirality, circular dichroism
and optical rotary dispersion will happen. The polarization state of light is affected, and
the reflectivity is different for the left hand circular and right hand circular light.

In our system, the head groups in the molecules are covalently connected and the
elongated lipid chains sticking to the head groups pack in a parallel pattern. Therefore
this system has a chiral smectic liquid crystal phase, which has found an application in
rapidly switching electrooptic shutters with response times within the microsecond range7
due to their ferroelectric properties. This system also has potential applications in many
other areas.

The structure can be made more complex by mixing the organic membrane with
inorganic additives. The synthesis of semiconductor nanoclusters and the exploration of
their nonlinear optical, electrical, photochemical, spectroscopic properties have attracted
a lot of interest lately due to their possible applications in solar cells, chemical and
biological sensors, and telecommunications. The crucial step towards their application is
the formation of nanocomponents into functional and desired nanostructures without
aggregation. The membrane we developed serves as a good template for nanocluster

deposition. With sulfur function groups, for example, certain metal ions can be combined,

40

and template control on nucleation and growth of metal ions can be achieved. The same
controlling effect can be achieved on calcium carbonate or calcium phosphate deposition
on membranes with phosphate groups or other biomineralization analog cases. The
morphology and chemical properties of the inorganic phase can be controlled through pH,
charge distribution, and lattice size and shape that can be adjusted by template design.
Chemical functionalities other than phosphate or sulfur function groups (such as
carboxylic acid, hydroxyl group, etc.) can be added to the polar region to introduce metal
affinity. Except for the nanoclusters mentioned above, metals also can be deposited to the
organic membrane to adjust the conductivity (such as Ag or Cu) or magnetic properties
(such as Co or Ni) of the system and improve the response of the system to outside
electrical or magnetic fields effects.

Biological recognization elements can also be added as customizable moieties. This
modification makes the system a potential biosensor with a specific biological element
that creates a recognition event and a physical element that transduces the recognition
event. Biological elements such as enzymes or their substrates, DNA strands, antibodies
or antigens are used as the analyzing sector. They distribute on the top membrane of the
system, and therefore have good exposure to the environment. They can recognize and
selectively combine the complementary component from the environment, and give
responsive signals. The responses of these recognization elements can be expressed

through structural changes and transported through the elongated chains connected to the

41

detector where it can be recorded and measured. Due to the multiple properties of the
system that we developed, different measurement parameters can be used such as current,
potential, conductivity, dielectric constants or the changes in the characters of light. If
fluorescent groups (such as cascade blue or arninometylecoumarin) are added to the polar
region, the molecular structure changes from specific binding can be identified directly

through fluorescence microscopy.

2.2.2 Non-polar region modification

The nonpolar portion includes the hydrophobic alkyl chain region, the electronically
and/or optically responsive region and the functionalized anchor region for attachment to
a substrate. The functionalized anchor is used to interact with a substrate or support. For
example, a thiol group can be added to the end of the alkyl chain so that the system can
be anchored on a gold surface. Thereby, the gold surface can be modified using the
system that we developed, and at the same time, this solid surface can help to further
stabilize the whole system. There are other substrates on which the systems can be
adhered using different functionalized anchors, such as silane for silica or alkyl chains for
plastic substrates.

The alkyl chains are the main component of the hydrophobic part. The alkyl chains give
order and control to the system through hydrophobic properties and the alkyl chain-alkyl

chain interactions. Due to their hydrophobic character, the alkyl chains pack in a parallel

42

pattern and self register at the polar-nonpolar interfaces to form the lamellar structure.
The hydrophobic alkyl chain region provides spacers for the different regions. The polar
groups, electronically and/or optically responsive regions and the functionalized anchors
are separated from each other by the alkyl chain regions. In addition, alkyl chains are not
conductive, therefore this layer can be the insulator layer between the polar region (which
can behave as conductors or semiconductors depending on the chemical functionalities
and the inorganic phases incorporated), and the electronically and/or optically responsive
region, which can be conductive due to its high conjugation. This complex structure can
be very usefirl in signal transduction in electronic equipment, and would be hard to

produce through other methods.

2.2.3 electronically and/or optically responsive region

The electronically and/or optically responsive region can provide the system
properties of fluorescence, conductance, chromism or liquid crystalline character. The
electronically and/or optically responsive region can be composed of the groups shown in
figure 2.2. The insertion of groups such as pyrene, diphenyl, diacetylene or
phenylacetylene makes an electronically and/or optically active region. These groups
have a common feature of high conjugation, which makes them very conductive. This
feature can be controlled by changing the geometry of the chains, as II-conjugation
depends on the relative orientation of the functional groups, and thus gives rise to a

molecular switch and other applications in electronics. Also, the conjugated system forms

43

excimers which brings a red shift through charge separation by intra-chain movement
of the photogenerated charge carriers. This system has the potential application in

long-wavelength laser production.

%

O
O

 

A B C D

Figure 2.2 Electronically and/or optically responsive groups A) pyrene B) diphenyl
C) diacetylene, D) phenylacetylene

Rigid polycyclic aromatic hydrocarbons that do not incorporate oxygen or nitrogen
atoms generally have large quantum yields of fluorescence. The hydrophobic pyrene
fluorophore is readily accommodated within the hydrophobic region. The pyrenebutaboic
acid conjugates have been reported to have exceptionally long excited-state lifetimes (t >
100 nanoseconds), and are consequently useful for time-resolved fluorescence
immunoassays and nucleic acid detections. The excimer-forrning process of pyrene can
be used to characterize membrane structural domains and their dependence on
temperature and other external disturbances.

The biphenyl group has the structure of two benzene rings joined by a carbon-carbon

44

bond. Energy changes in the system such as UV light excitation, or thermal energy
exchange will cause conformational changes of the biphenyl group, which will lead to
packing density and pattern adjustments of the alkyl chains. The torsion angle (‘1’) is
connected with the energy in the system. The energy curve reaches its minimum at ‘l’=45°
and its maxima with 7 k1 above the minimum at ‘l’=90° and ‘I’=0° (figure 2.3). The
biphenyl group has a strong absorption in the UV range but weak fluorescence. It
-electron delocalization reaches its maximum when the two phenyls are in a coplanar
arrangement, and the delocalization leads to a bathochrornic shift in the absorption
spectra. Thus the biphenyl groups respond to optical stimuli differently, and give

information about their conformation structure.

Q Q w=0°

w=90°

 

Figure 2.3 Biphenyl conformation (adapted from reference 9)

The diaceylene group is not polycyclic, however, due to the high conjugation of the

repeating double and triple bonded alternating structure it forms after polymerization.

The diacetylene introduction can bring several unique properties into the system. First,

45

Polydiacetylene compounds have very unique chromatic properties. Polydiacetylene
compounds demonstrate color changes between blue forms and red forms with exciton
absorption peaks at 640 nm and 560 nm under environmental perturbations such as
solvent, pH, mechanical and temperature changes. Second, Polydiacetylene (PDA) has a
high nonlinear optical (N LO) coefficient. When the intense light goes through a material
with diacetylene moieties, significant non-linear effects such as second harmonic
generation, four-wave mixing and frequency doubling appear. These effects can be
applied to a lot of different non-linear optical devices. Thirdly, PDA is photo-linkable
material. Under the irradiation of heat or UV light, PDA is very easy to polymerize and
forms another polymerized layer to make a two dimensional system with additional
stability. Last, due to its highly conjugated system PDA has high electronic conductivity.
The polydiacetylene chain is the primary candidate for molecular wires for integrated
nano-sized electronic circuits.

Phenyl acetylene is another possible functionalized group that can be inserted in the
non-polar part of the system. It is commonly used as a precursor for the synthesis of the
polymer polyphenylacetylene. Polyphenylacetylene has non-linear optical properties, and
has potential use in semiconductors, electrophotographic photoreceptors and chemical
sensor devices'o. polyphenylacetylene can be viewed as a combination of multiple bonds
and aromatic rings. Therefore polyphenylacetylene has optical responses through

configuration changes between the cis and trans forms of double bonds and the

46

orientation change of the phenyl ring. The polymerization also brings additional
stability.

The multifunctional polymer-secured self-registering molecular arrays system
includes four different parts—the polar recognition and chemo-responsive region, the
hydrophobic alkyl chain spacer region, the electronically and/or optically responsive
region and the functionalized anchor region. All these different parts were connected
together to form self-registering polymerized structures with functionalized
multiple-layers. This system has potential applications in many important areas such as

biosensors, electronic or magnetic equipments and optical communications.

47

BIBLIOGRAPHY

l Bnlst, M; Kiely, C; Colloids Surf A, 2002, 202, 175

2 Whitesides, GM; Grzybowski, B; Science, 2002, 295, 2418

3 Lindsey, JS; NewJ. Chem, 1991, 15, 153

4 Huie, JC; Smart materials and structures, 2003, 12, 264-271

5 http://star.tau.ac.i|/~andelman/moked_en.html

6 Bovery, FA, Winslow, FH, Macromolecules, 1979, Academic Press, New York

7 Giuseppe S, Carlo Versace, Nicola Scaramuzza, and Vmcenzo Bruno, J. of app]. Phy.,
2002, 92(7), 3630

8 http://www.probes.com/handbook/sections/ l 302.html and references included
9 Brock, CP; J. Res. Natl. Inst. Stand. Technol., 1996, 101, 321

10 (a) Neher, D.; Wolf, A.; Bubeck, C.; Wegner, G. Chem. Phys. Lett. 1989, 163, 116. (b)
Bearzotti, A.; Foglietti, V.; Polzonetti, G; Iucci, G; Furlani, A.; Russo, M. V. Mater: Sci.
Eng. B 1996,40, 1.‘ (c) Yang, M. J.; Xu, J. L.; Hu, QM. J. Photochem. Photobio. A -
Chem. 1995, 85, 147. (d) Caliendo, C.; Verona, E.; Mico, A.; Furlani, A.; Iucci, G; Russo,
M. V. Sensors Actuators B 1993, 15-16, 288.

48

CHAPTER 3

DESIGN OF THREE POLYMER-SECURED SELF-REGISTERING MOLECULAR
ARRAYS SYSTEMS

ABSTRACT

Three systems have been developed based on an idealized model for systems composed
of polymer-secured self-registering molecular arrays with four components: the polar
recognition and chemo-responsive region, the hydrophobic alkyl chain spacer region, the
electronically and/or optically responsive region and functionalized anchor region for
attachment to the substrates. In system 1, there is negatively charged phosphoric acid in
the polar region for template-regulated calcium phosphate deposition to make a
biomimetic material with potential application as bone filler or scaffold of bone tissue
engineering. Both systems 2 and system 3 have sulfur function in the polar region for
extensively ordered nanoparticle structure fabrication using soft cations such as cadmium
and zinc. In system 3, the electronically and optically responsive region contains
diacetylene groups which have unique chromatic properties that include optical and
electronic responses once polymerized. Photopolymerization of the diacetylene groups
also gives the system additional stability. Potential uses include the fabrication of sensors,

detectors, electronic components, bioreactor compartments, etc.

49

Here we describe the preparation and characterization of the organic structures that
serve as templates for the directed deposition of inorganic systems. As explained
previously, our ideal target system has a polar recognition and chemo-responsive region,
hydrophobic alkyl chain spacer regions, an electronically and/or optically responsive
region and a functionalized anchor region for attachment to a substrate. The electronically
and/or optically responsive region and the ftmctionalized anchor region are important for
transducing signals and the fabrication of hybrid materials.

The organic templates we designed are polymer-secured self-registering molecular
arrays. Each unit is amphiphilic with polar heads and nonpolar alky chains. The polar
heads are connected and interact with more polar solvents and the chains in the nonpolar
regions assemble to form regular arrays. This self- registering method originates from
nature. It uses entropy as its driving force for orientation and saves a lot of endeavors.

Three different systems were designed and prepared based on this general architecture.

3.1 Polymer-secured self-registering molecular arrays system with phosphoric acid

group in the polar region and saturated hydrocarbon chains (system I)
Polymer-secured self-registering molecular arrays system with phosphoric acid groups

in the polar region and saturated hydrocarbon chains has two structural and functional

regions (Figure 3.1).

50

 

Hydrophilic, chirality for optical activity, polyamide as
polymerizable moieties, hydroxyl group for fillther
functionalization, phosphoric acid groups for calcium
phosphate deposition, negatively charged

 

 

Hydrophobic alkyl chain

Hydrophobic, parallel alligned to give stability and order
region to structure

 

 

 

 

Figure 3.1 Table illustration of system I

System I has the phosphoric acid groups introduced into the polar region. This results
in a biomimetic material with potential application as bone filler or scaffold of bone
tissue engineering. The amphiphilic molecules self-orient at the interface of polar and
nonpolar solvents. “rub the additional stabilization of polymerization, it forms a robust
two dimensional template resembling collagen templates in bones.

The phosphoric acid group is negatively charged, and therefore it combines metal ions
such as calcium through ionic interactions. Calcium phosphate has been used extensively
for bone replacement due to its similarity to the mineral component of bone'. Thus
calcium phosphate was chosen as the inorganic phase for our system. It makes the surface
of the template biocompatible and bioactive so that the material can be integrated into the
tissue by the same processes of healthy bone remodeling. The main draw back of bulk
calcium phosphate is its brittle nature and poor mechanical properties, which can be

greatly reduced by the self-assembled polymerized templatesz.

The structure of this hierarchical system can be controlled in a lot of different ways.

51

The thickness of the template can be chemically tailored by changing the length of the
fatty acid chain. The charge density and the morphology of the inorganic phase can be
controlled by conditions such as the concentration of the polymer, the pH of the solution
and the temperature. The chemical structure of polymer-secured self-registering
molecular arrays with phosphoric acid groups in the polar region and saturated

hydrocarbon chains is shown in figure 3.2.

06

8 8 8
see:

Figure 3.2 Polymer-secured self-registering molecular arrays with phosphoric acid groups
in the polar region

52

\JJSL)
C)

 

3.2 Polymer-secured self-registering molecular arrays with thio-ether groups in
the polar region and saturated hydrocarbon chains in the nonpolar region (system
II)

The second system we have developed featured two regions, the polar recognition and
chemo-responsive region and a hydrophobic alkyl chain region (figure 3.3). This system
has polyarnides as polymer linkages in the polar region. As a result, the polar region is
hydrophilic with strong dipoles. Hydrocarbon chains connected to the polymer backbone
are in parallel arrays, therefore the system has smectic liquid crystal properties.

Sulfur functional groups in the polar region have a high affinity towards metals such as
gold or mercury’. Therefore, systems like this can modify the metal surface by changing
the conductivity, morphology and polarity. They have potential applications in electrode

modification in electrochemistry, corrosion protection and new material manufacturing.

 

Hydrophilic, neutral, chirality for optical activity, polyamide
as polymerizable moieties, thio-ether group binding metals
to make conductive or magnetic, hydroxyl group for further

functionalization

 

l'l_\,r"d1‘0p110blC alkyl Chain Hydrophobic, 15 methylene groups, parallel alignment gives
stability and order to structure, insulator, affinity for the
region plastic surface

 

 

 

 

Figure 3.3 Table illustration of system 11
The hydrophobic alkyl chain region can also serve as anchoring group. The
hydrocarbon chains are hydrophobic, therefore can be attached to the plastic surface. The

nonconductive plastic surface can now be made conductive and be used in electronic

53

elements by combining metal ions to the sulfur function groups in the polar region with
following reduction or counterion combining. Electronic conducting components with
plastic substrates can be manufactured using this method.

The saturated fatty acid chain is straight comparing with the unsaturated ones.
Consequently, the chains are packed more tightly and induce different properties in the
system, e.g. the crystal lattice, the melting point, etc. The structure of the
polymer-secured self-registering molecular arrays with thio-ether groups in the polar

region and saturated hydrocarbon chains in the nonpolar region is shown in figure 3.4.

3.3 Polymer-secured self-registering molecular arrays with thio-ether groups in the
polar region and diacetylene moieties in the nonpolar region (system III)

System 111 has polymer-secured self-registering molecular arrays with thio-ether
groups in the polar region and diacetylene moieties in the nonpolar region as illustrated in
figure 3.5. It features four major structural and functional regions and can be developed

into a variety of sensors or electronic devices4.

3.3.1 Polar region of system 111
Unlike self-assembled systems, where the units connect with each other through
electrostatic forces, Van der waals interactions, It -stacking or hydrogen bonds and the

systems are too delicate for practical applications, this strategy of connecting the units

54

with covalent bonds gives these systems more stability and mechanical strength. The
polyamide moieties are biological analogues of polypeptides and should consequently be
biocompatible with cells and biomolecular systems. The electrons from the double bond
in the carbonyl group migrate to the bond between the C and the N producing partial
positive and negative charges on the N and 0. This dipole makes the polymer very
polar and hydrophilic. The regular repeating unit helps to space and align the side chains.
The distance between the monomeric units is close due to the short covalent bond length.
This polymerized system is much more stable and densely packed than the traditional
self-assembled system.

System 111 has two chiral centers in each unit in the polar region. The chirality makes
the system optically active. It can rotate the polarized light, and cause circular dichroism
and birefringence. As most important biological molecules are chiral, the stereochemical
sites of system III can selectively bind the biomolecules according to their chirality and
then be used for biomolecular detection and purification. Chirality can also bring
biological activity to these systems.

In system 111, the elongated side chains lie in parallel alignments and orient
perpendicular to the chiral polymeric polar region. Therefore our system is a liquid
crystalline material with stabilized chiral smectic C phases. Liquid crystals have an
orientational order and in some cases a weak positional order, and they also flow like

liquidss. They are called soft materials because of their combination of orientation and

55

fluidity. Since the orientation and hence anisotropic properties are carried by liquid,
liquid crystals can be manipulated very easily by changing electrical, magnetic or optical
fields. This gives them big electro-optical, magneto-optical and opto-optical effects. The
electro-optical effects of liquid crystals have already been widely used in the display
industry. The magneto-optical effects of the liquid crystals can be used in recording
media, and the strong opto-optical effects can be used in non-linear optics, which is
important for all-optical switching and other devices in future optical information

technologies.

ll 3 Fl 8 11,
“N/\/:(\"/ V—XY \/—//\(\r ‘m
H \x
O O O ‘
OH
O OH O OH 0
0 O 0

Figure 3.4 Self-registering polymeric system with sulfur functions in the polar
region

56

 

Hydrophilic, neutral, polar, chirality for optical activity,
polyamide as polymer moieties, hydroxyl group for further
functionalization, sulfur functions binding metals to make
system conductive or magnetic or to manufacture

nanostructure

 

 

Spacer region Hydrophobic, with 9 methylene groups in each
hydrocarbon chain, parallel alignment to give stability and
[ilecu‘mllt‘lllh' mid/01’ order to structure, function as spacer to separate regions

and insulator layer

 

optically responsive _
Diacetylene brings fluorescence, conductance. chromism,
revion nonlinear optical properties to system, can be
F photopolymerized

 

Hydrophobic alkyl chain

 

Hydrophobic, with 7 methylene groups in each
spacer region hydrocarbon chain

 

 

 

 

Figure 3.5 Table illustration of system 111

The liquid crystal is classified by the orientation of its axis and molecular order.
Among all these different species, the smectic phase is the most ordered mesophase. This
phase has not only orientational order, but also to some extent positional order. The
smectic C phase has the elongated side chains tilted with respect to the perpendicular axis
of the main chain. This gives the smectic C phase an optically biaxial property. Chiral
smectic C (smectic C") phases have the same structure as smectic C phases except for
their chirality. Materials with smectic 0" phases can be used in rapidly switching
electrooptic shutters with response times within the microsecond range6 because of their

ferroelectric properties.

y- Hydroxyl groups makes system 111 more hydrophilic and polar. The hydroxyl
groups can be modified to form others or esters, which allows for further
functionalization of the polar region. Another modification that can be done to the polar
region of the system is to incorporate inorganic phases. Functional groups in the polar
region provide sites for ordered deposition of ions and crystal growth. For instance,
certain cations can be bound in the polar region in an induced pattern and then reduced or
combined with anions to form salts. The inorganic phase might import different
properties such as conductance or semi-conductance, magnetism (in the case of nickel or
cobalt), fluorescence (in the case of cadmium sulfide or selenide) and nonlinear optical
properties to the system. In addition, labeled or unlabeled ligands, enzymes, antibodies,
antigens, receptors, nucleic acids, oligonucleotides or other biomolecules can be
incorporated and used to measure, monitor or study biologically important processes.
With different functional groups and conditions such as pH, temperature and
concentration, the physical and chemical properties of the inorganic phases can be
adjusted.

In system 111, thio-ether groups in the polar region have a high affinity for certain metal
elements. The polymer-secured self-registering molecular arrays closely pack at the
interface to provide the template for the deposition of these metal ions and for

nanostructures and other ordered arrays to grow.

58

3.3.2 Hydrophobic alkyl chain spacer region of system 111

The alkyl chain is the main component in the nonpolar region. The alkyl chains are
connected to the polar region through ester bonds. Under multiple effects such as head
group arrangement, hydrophobic interactions and interactions between the chains, they
align in parallel arrays to give stability and order to the structure. The alkyl chains are
hydrophobic and insulating. The alkyl chain region can be viewed horizontally as spacers
between the polar region and the electronically and/or optically responsive region, which
we will introduce later. The thickness of the layer can be tailored by changing the number
of ethylene groups. In system 111, there are nine ethylene groups in the first hydrophobic
alkyl chain region and seven ethylene groups in the second. The hydrophobic chains
also prevent polar molecules, ions, etc. from diffusing across the membrane, and

therefore help to stablize the inorganic phase.

3.3.3 Electronic and/or optically responsive region of system 111

The electronically and/or optically responsive region of system 111 has the diacetylene
groups. The diacetylene moiety raises many research interests due to its ability to form
conjugated double-triple alternating bonds under heat or UV-light irradiation, the
quasi-one-dimensional highly conjugated system which gives it high electrical
conductivity, fluorescence, high third-order nonlinear optical coefiiciency, and chromatic

properties. These special properties enable it to be used in a variety of applications in

59

electronics and optics. Generally, when the adjacent diacetylene residues in a crystal
lattice are stacked parallel at a distance of about 5 A and the angle between monomers
with stacking axis is about 45°, diacetylene moieties can be polymerized (figure 3.6) 7.
Therefore in order for the diacetylene to photopolymerize, the hydrocarbon chains have
to be well parallel ordered. In the system 111 that we have currently developed, the long
fatty acid chains with diacetylene residues automatically arrange in parallel order and the
polymer backbone connects the hydrocarbon chains within a polymerizable distance. The
diacetylene moiety is in the middle of the polar region and the fatty acid chain, this
location helps protect the diacetylene part from oxidative degradation and improves its
processability. After photopolymerization, a system with two folds of linked layers is
formed. The crosslinking of diacetylene improves the mechanical properties of the
system.

Our system 111 has potential applications as elements in electronic devices in which the
conductive diacetylene layer is separated from the conductive or semi-conductive layer
with inorganic phases by an insulating alkyl layer. The thiclcness of this insulating layer
can be controlled by the length of the intervening alkyl chains. Environmental stimuli can
cause packing pattern alterations in the polar region. The signal will be transferred along
the side chains and be detected at the responsive region with the diacetylene moieties

through color, fluorescent or conductance changes.

60

diacetylene
(DA)

 

 

polydiacetylene
(PDA)

 

“Of-tolpouon An

 

spacer

“—1—3.

Figure 3.6 The polymerization of diacetylene (adapted from reference 7)

Another important potential application of this system is its use as masks in
photolithographic methods, which are widely used in integrated microelectronic circuit
printings. The diacetylene groups crosslink under the irradiation of lasers, and therefore
protect the regions that are covered by the material. After the laser treatment and removal

of the protecting material, the desired pattern can be achieved. The chemical structure of

61

the polymer-secured self-registering molecular arrays with thio-ether groups in the

polar region and diacetylene moieties in the nonpolar region is shown in figure 3.7.
.\§ - n\/—S . nv—s . x”
0 O O t
0 o o
0 O 0

 

 

 

Figure 3.7 Polymer-secured self-registering molecular arrays with thio-ether group in the
polar region and diacetylene moieties in the nonpolar region

62

BIBLIOGRAPHY

1 LeGeros, RZ; LeGeros, JP; Key Engineering Materials, 2003, vols. 240-242, 3-10

2 CA. Vacanti et al, New Engl. J. Med, 2001 344:1511

3 (a) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (b) Porter, M. D.;
Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (c)
Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 47, 437. (d) Ulman, A. An
Introduction to Ultrathin Organic Films: From Langmuir-Blogett to Self Assembly;
Academic: Boston, MA, 1991

4 (a) Rudolph, A. S.; Calvert, J. M.; Schoen, P. E.; Schnur, J. M. In Biotechnological
Applications of Lipid Microstructures; Gaber, B. P., Schnur, J. M., Chapman, D., Eds;
Plenum Press: New York, 1988; Vol. 238, pp 305-320.(b) Schnur, J. M.; Price, R.; Schoen,
P.; Yager, P.; Calvert, J .; Georger, J.; Singh, A. Thin Solid Films 1987, 152, 181-206

5 (a) Collings, P. J; Pate ,J. S.; Handbook of Liquid Crystal Research,1997 (b) Chigrinov,
V. G Liquid Crystal Devices, Adtech Book Co Ltd,1999 (c) Khoo, 1; Wu, S; Optics and

Nonlinear Optics of Liquid Crystals, World Scientific Publishing ,1992 (d) Sonin, AA;
The Surface Physics of Liquid Crystals, published by Gordon and Breach, 1995

6 Giuseppe S, Carlo V, Nicola S, and Vlncenzo B, J. of app]. Phy., 2002, 92(7), 3630
7 http://stm.lbl.gov/~kopta/figures/web5.html
8 Al-Sheikhly, M; McLaughlin, WL; Kovacs, A; Wojnarovits, L; Christou, CA; Christou

A; Lewis, DF; SPIE Proceedings, 1997, Vol. 3005 184-192

63

CHAPTER 4

PREPARATION AND CHARACTERIZATION OF THE THREE POLYMER-
SECURED SELF-REGISTERING MOLECULAR ARRAY SYSTEMS

ABSTRACT

The strategies for making the polymer-secured self-registering molecular arrays are
discussed. There are 2 such strategies. In the first one a suitably functionalized primary
amine is added to a 5-substituted-2(5H)-furanone to form an intermediate 3-amino- Y
-lactone which is then polymerize to form substituted B -polyamino acid. In the second
strategy an aminothiol is reacted with the furanone to form an intermediate
(2-aminoethyl)thio—substituted y-lactone which is then polymerized to form a scaffold
containing alternative thio-ether and amide linkages. Four systems using one or the other
of these strategies were prepared. System I which was prepared with strategy 1 mimics
phospholipid biomembranes. System 11 and III which were prepared with strategy 2
served as scaffold for forming extensively ordered nanoparticale arrays using metal salts
such as CdSe, CdS, ZnSe and ZnS. System 111 has diacetylene groups incorporated in the
long fatty acid chain. Polymerization of these allow them to be used as electronically and
optically responsive systems. The high long range order of polymer IV was explored with

X-ray diffraction method.

4.1 General strategies for polymer-secured self-registering molecular array system
preparation

There are two general strategies for preparing polymer systems as the backbone of the
polymer-secured self-registering molecular arrays. In the first strategy, a suitably
functionalized primary amine was added to a 5-substituted-2(5H)-furanone to form an
intermediate 3-amino— Y -lactone which is then polymerize to form substituted 13

-polyamino acid. This strategy is illustrated in scheme 4.1.

O 0
R2 R2 R2
i o * R2NH2 ——-» o ———> ~ * N * N * N“
* * * * ci-lo * O
* HO
Nl'lR2 0R1 0R1 0R1
R10 R10

Scheme 4.1 General strategy 1

In the second strategy an aminothiol is reacted with the furanone to form an
intermediate (2-arrlinoethyl)thio-substituted y-lactone, which was then polymerized to
form a scaffold containing alternative thio-ether and amide linkages (scheme 4.2). In both
cases, a choice of the substitutents on the furanone allows the incorporation of critical
elements of the design such as the functionality in the electronically and optically
responsive region, the type of anchor group, and the separation of these elements by the

hydrophobic regions. The choice of the amine allows the incorporation of critical

65

elements at the polar interface region.

0 O H H H
‘xNNWNV—S" ON/‘8 ON“
+ ——-> O H * e
0 HS NH2 . —_> OH OH OH

t ' R0 R0 R0

SH
R0 ( R0

NHz

Scheme 4.2 General strategy 2

In both strategies, a substituted 2(5H)-furanone was used as the Michael acceptor in
the preparation of the polymer-secured self-registering systems. Generally, an equimolar
amount of a primary amine or aminothiol was added to the suitably derivatized furanone
under mild conditions to form the intermediate 3-substituted y-lactone, and the
temperature was then raised to effect polymerization.

The three critical elements in the real designs of these systems are:

(l) The furanone — (S)-5-hydroxymethyl-2(5H)-furanone, which is optically active.
Michael addition to the furanone is generally stereospecificl and another chiral center is
therefore introduced. Due to the stereohindrance of hydroxymethyl groups, the addition
takes place on the opposite face to that containing the hydroxymethyl groups. Take
strategy 2 for example, as described in scheme 3, the chiral center in furanone has the S
configuration. In the expected scenario for the addition of an aminothiol the newly

introduced chiral center would have the absolute configuration of S.

66

 

NH2

Scheme 4.3 Mechanism of Michael addition of an aminothiol to the furanone

(2) The hydrophobic alkyl chain region (usually as an acid chloride) that
incorporates the reporter elements, such as the diacetylene group, and anchoring
functionalities.

(3) The headgroups/interface component which is a primary amine or an aminothiol.
These may contain additional functionalities such as charged groups.

System I was prepared using strategy 1 and system 11 and III were prepared using

strategy 2.

4.2 Preparation of a stabilized phospholipid biomembrane monolayer mimetics
(system I)

Phospholipid biomembrane mimetics have been widely studied and have many
significant scientific and technological applications. They are used as interfaces between

living systems and man-made materials. Desirable properties of biomembrane mimetics

67

include the ability to inhibit the denaturation, degradation or precipitation of proteins,
and the prevention of cell transformation on contact with nonbiological materials.
Biomembrane mimetics should also display hemocompatibility, that is the ability to stop
blood cells from depositing, lysing or clotting on contact with foreign surfaces. Other
applications include making biocompatible implants or artificial tissues. Biomembrane
mimetics are also important for the fabrication of sensors for biological or biochemical
application where cells, antibodies, antigens, receptors and enzymes must be presented in
a context that resembles the surface chemistry of a living cell. Biomembrane mimetics
can be used to coat surfaces in the fabrication of bioreactors in which the living cells
anchor to a support where in-vivo conditions were imitated.

We designed a polymer system which has the surface properties of a phosphatydic acid
monolayer system in which the noncovalently associated glycerol components are
replaced by a polyamide chain (figure 4.1). There are several advantages to the design of
this system:

1 The functionality in the head groups of our system is the same as in phospholipids and
proteins (should result in biocompatibility).

2 The covalent connection of the headgroups gives the system a stability and robustness
that cannot be matched by ones based on noncovalent interactions. Many applications
that require conditions where the phospholipids are not stable can now be explored.

3 Flexibility: The alkyl chain and head group regions can be incorporated without

68

difficulty.
4 General utility. The head groups can be modified chemically or biologically.
5 Robustness: The system is easy to process as self-standing films or as supported layers
without risk of breaking.
6 Higher structural integrity than phospholipids where fatty acid chains can be
heterogeneous
7 Chirality in the polar head group region enabling the use of polarization techniques and

allowing enantioselective discrimination

o o o o o o' o\ p - - -
\ /\ - $1 - \\P/\ - \P\ - 9 ’OH 9 IOH Q ’OH
0’ 0 d O O 0’ 0 0'13 O-P O'P

«asses-s8 8 8 8
. margin

0

I

Figure 4.1 Comparison of a natural phosphatidic acid self-assembled system (left) and the
stabilized phospholipid biomembrane monolayer mimetic I (right)

In addition to the attributes described above, the biomembrane mimetic system we

69

designed has added advantages of simplicity of preparation and ease of purification.

The synthetic strategy to prepare the biomembrane mimetic system (system I) is

illustrated in scheme 4.4.

HO

 

\ ,/o
C15H13COCI OH
base
H O
O O
2

o 9 HQ -
OJP’OH O‘P’OH O‘P’O

Li‘s: EZLEZK
c><3 C)() C)() C)<D

Scheme 4.4 Synthesis of system 1

1n the synthesis of system 1, (S)-(-)-5{hydroxymethyl)-2(5H)-furanone (l) is reacting

with hexadecanoyl chloride to form the intermediate Michael acceptor (2), which is then

reacted with 2-aminoethyl dihydrogen-phosphate to eventually form the desired polymer

system I— the phosphatidic acid mimetic.

70

The potential applications for phosphatidic acid mimetics are many, such as making
man-made materials (i.e. plastic or metals) biocompatible for use in clinical applications,
serving as elements or matrices in biosensors, or as coatings for bioreactors as mentioned
above. Here we explored the utility of the phosphatidic acid mimetics l as a primary
foundation or template for the ordered deposition of calcium phosphate to model bone
formation. This process is illustrated in figure 4.2. One common biological strategy
employed in biomineralization is controlling the location, size, composition, and shape of
mineral crystals by the lipid mesophasesz. Generally the process begins with the capture
of calcium ions by the negatively charged phospholipid layer, followed by subsequent
capture of phosphate ions, layer upon layer, as illustrated in figure 4.2. Because of its
similarity to phospholipid membrane systems, the synthetic stabilized phospholipid
membrane mimetic I should serve as an efficient template for the promotion of calcium
phosphate deposition.

Polyethylene was used as a substrate on which the polymer system I was coated. On
contact of an aqueous solution of I with polyethylene, the fatty acid chains (being
hydrophobic) will attach to polyethylene with the polar surface facing outward toward the
aqueous medium. The coated plastic was then exposed to calcium and phosphate ions
under equilibrium conditions for 7 to 35 days. The experimental conditions were arranged
so that the modified plastic surface was never contacted by preformed calcium phosphate

particles. This was accomplished by separating the treated plastic from calcium

7]

phosphate with a membrane. Thus, the properties of the inorganic phase (calcium
phosphate in this case) such as density, morphology, growth orientation etc. will be

controlled by the template made fi'om the phosphatidic acid mimetics.

3' —-|-O O O o

 

 

 

 

 

 

Plastic Substrate

Figure 4.2 Illustration of the phosphatidic acid mimetics’ template control over
calcification

4.3 Preparation of nanoparticle arrays on self-standing and unsupported
two-dimensional self-registering polymeric systems (system [I and III)

Nanoparticles are microscopic particles with all three dimensions being within the
nanometer scale (i.e. between 1 to 100 nm). On this scale, since the size of the
nanoparticles is smaller than the Bohr radius of an electron-hole pair, quantum
confinement effects come into play. The most desirable types of nanoparticles include

cadmium and zinc salts because they are semiconductors. Their nonlinear optical,

72

electrical, photochemical and spectroscopic properties have attracted much interest due
to the possible applications in solar cells, chemical and biological sensors and
telecommunication.

Nanoclusters have very high surface areas. This feature gives them attractive properties,
such as high activity, with potential applications in catalysis and chemo-responsive
sensors, but at the same time it also makes them aggregate very easily and lose their
activity. Processing of nanoparticles is very problematic and challenging because of their
poor mechanical strength and stability. Dispersion of nanoparticles in polymers has
solved the problem to some extent. The two most common approaches include blending
and in situ polymerization3. Direct blending mainly uses the shear force between the
particle and polymer melt or solution. This method is relatively easy, but ofien
unsuccessful in achieving efficient nanoparticle dispersion. In situ polymerization
methods were adopted because nanoparticles are more easily dispersed in the monomer
solution which can be polymerized afterwards. However, this method is restricted to

certain polymerization procedures due to the particular requirement of this approach.

4.3.1 A two dimensional self-registering polymeric system without an electronically
and optically responsive region (system [1)
The first of the sulfur functionalized self-registering polymers we designed as a

template for the controlled deposition of inorganic nanoparticles is shown in figure 4.3.

73

The polymer can form a two dimensional stabilized membrane with specific orientation
that could be used to incorporate the zero-dimensional nanoparticles in ordered arrays on

the hydrophilic surface and efficiently protect them fiom aggregating and precipitating

(figure 4.4).

\‘\ n S n S “x
~.N/\/S \/_ \/_ ‘~.\
H o o o \
OH
0 OH 0 OH 0
o o 0

Figure 4.3 Polymer-secured self-registering molecular arrays with thio-ether group in the
polar region (system 11)

74

 

O O
o O O
o O O
O
O o
O O
20 system 00 ”Stems

(template)

 

w

Ordered array

Figure 4.4 Ordering effects of the two dimensional template on nanoparticles

System II has sulfur functional groups which have strong affinities for certain metals
such as cadmium and zinc in its repeat unit. Individual sulfur atoms can be sequestered at
regular points on this 2-dimensioanl system and each can serve as the nucleus for the
slow, directed and controlled growth of individual nanoparticles. The specific
arrangement of sulfur functional groups in the template is transferred to the nanoparticle,
and organized arrays of the nanoparticles will be formed. This method is more efficient
compared to the blending method, which only uses the shear force between the particles

and the solution. The template is very accessible by a concise synthetic scheme. It can

75

be prepared as a free-standing system or as a supported layer. As a consequence, a
two-dimensional nanoparticle array on a processable membrane system is formed.

The self-registering polymer with saturated alkyl chains and sulfur functions in the
polar region (system II) was formed through the condensation of the Michael acceptor 2

and 2-mercaptoethylamine shown in scheme 4.5.

H H H
0 ~.
“NA/S N\/——S N\/'_S Na‘x‘
O i H O O 0 ‘\
OH
O O OH O OH O
O / \ —> O O O
+ HS NH2
2 II

Scheme 4.5 Synthesis of sulfur functionalized self-registering polymer 11

4.3.1.1 An unsupported membrane made from polymer II

The preparation of calcium selenide arrays on the polymer template II is illustrated in

figure 4.5. The solution of II was deposited on top of the cadmium perchlorate aqueous

76

solution. After the solvent evaporated, the polymer formed a membrane floating on top
of the aqueous solution. The hydrophilic head groups or the polar region of the polymer
were immersed in the semiconductor solution, and the hydrophobic alkyl chains pointed
to the air in parallel arrays. After incubation, ordered arrays of cadmium perchlorate
nanoparticles with wurzite structure were formed on the hydrophilic polar surface of II
under the control of the sulfur groups. The stabilized two-dimensional membrane with
cadmium perchlorate as the inorganic phase modification was formed on top of the water.
In step 1 indicated in figure 4.5, perchlorate ions were replaced by selenide in sodium
selenide solution. In the second step the membrane was removed from water to form the
unsupported membrane with ordered arrays of cadmium selenide nanoparticles
incorporated in the polar region. The cadmium/zinc chacogenide nanoparticles can all be
deposited on the unsupported membrane made from polymer II using this method.

The cadmium/zinc chalcogenide crystals are semiconducting. They are anchored on the
polar region of the polymer, which has the long insulating hydrocarbon chain. Therefore,
the hierarchical structure is electronically dipolar. In addition, the quantum dots formed
with cadmium/zinc chalcogenide have optical properties such as nonlinear optical effects
and fluorescence. The head groups are covalently linked by polyamide bonds, so the
membrane formed is robust and can be easily removed from the top of water and used in

different ways, such as surface coating, conductance converters and optical devices.

77

 

 

 

 

~ § 1‘; 3..- f:-—{ J St 1 fizz; 3; 1‘ {W l 3
Il l‘ ‘ l stttts’e 2 lg
MKS: ‘e.}.\ Vii. Kt), . . f.‘ .0 . . ,.,‘_ A , I.-. ; i \
i L i i i 1 l . . I ‘1 ‘ i . I . i Pry:' xix/:5 :v‘“J:-.v—:V.b\ 1.1:
00.... 50000000 \B‘OK‘”) «13;,
rs
l .V . . \J o 0
2+ 2- 2 +
x,‘ Cd ."’ C104 0 3 -Na

Figure 4.5 Preparation of an unsupported membrane made from polymer 11

4.3.1.2 A freestanding film made from the sulfur functionalized self- registering
polymer II

A freestanding film of the sulfur functinalized self-registering polymer has also been
prepared as illustrated in figure 6. The standard procedure for preparing freestanding lipid
membranes was adopted. In this method, a concentrated solution of the polymer was cast
across a fine metal wire loop with diameter of 1 mm. Afier the solvent evaporated, the
metal loop with the polymer membrane was placed in water overnight. In the presence of
water, the amphiphilic molecules in the membrane should rearrange to form a bilayer
arrangement with the polar heads pointing toward the aqueous phase and the hydrophobic

hydrocarbon chains oriented inwards between the layers to interact with each other.

78

Self-assembled lipid freestanding membranes derived from phospholipids can
normally only be 0.1 mm2 in area at the maximum. They are fragile and the lifetime is
normally no more than 24 hours. The membrane system we prepared with polymer II has
a much larger area and a much longer lifetime (7 days at least) due to the stablization of

polyamide linkages in the polymer.

.0. 09 0.0.9.:

 

, v 1] 505...

lhlJ‘“

09.0

 

Q— Soluble ions O—Nanoparticle ions 0 — Metal ions

Figure 4.6 Preparation of freestanding membranes made from II

Cadmium or zinc chalcogenide particles were then deposited on the membrane in
ordered arrays. The metal ions attached to the top of the membrane to form crystals,
which by virtue of their optical properties had the wurzite structure.

This method of freestanding membrane preparation is straightforward, easy, flexible,
cost-saving and fast. This is in contrast to the Langmuir-Blodgett method, which is

necessary for the preparation of films from typical self-assembled systems. The

 

Av

I,

self-registering polymeric membrane preapared from polymer II is robust, well ordered
and very amenable to inorganic mineral modifications.

A similar method was adopted to make pinhole supported self-standing membranes. In
this method, suspension of polymer II was painted across a pinhole in a polyethylene
sheet. After the solvent was evaporated, the membrane was put in water to form a bilayer.

The membrane can then be modified with inorganic phases by dipping into solutions.

4.3.2 A two dimensional self-registering polymeric system with electronically and
optically responsive region (system III)

The third system we designed and prepared is illustrated in figure 4.7. The alkyl
chain in this system incorporates a polydiacetylene functionality. This self-registering
polymer shares the same backbone with the first sulfur functionalized self-registering
polymer II described in the previous section. The diacetylene functional groups can
polymerize by irradiation with heat or UV radiation. This will further stablize the system
and greatly modify the properties of the system at the same time.

Polydiacetylene compounds have very unique chromatic properties. They have two
distinct spectroscopic phases, blue forms and red forms with exciton absorption peaks at
640 nm and 560 nm. Under environmental perturbations such as structural changes in the
polar region, solvent, pH, mechanical and temperature changes, polydiacetylene
functionalities can demonstrate color changes from blue to red. This property can be used

in the investigation of membrane interactions in biological studies and sensors for

80

different chemicals. Polydiacetylene has high nonlinear optical (NLO) effects which
will further modify the properties of the light beam passing through the material
including the phase, frequency and state of polarization after nanoparticle modification in
the polar region. In addition, with doping, polydiacetylene can be very conductive. This
will enable the fabrication of electronic devices with multiple layers of different
conductivity.

To make the polymers, 10,12-docosadiynedioic acid (scheme 4.6) or
10,12-tricosadiynedioic acid (scheme 4.7) was transferred into the corresponding acid
chloride through reaction with oxalyl chloride. The acid chloride reacts with (S)-(-)-5-
(hydroxymethyl)-2(5H)-furanone to form the Michael acceptor monomer. The monomer

was then mixed with 2-aminoethanethiol and primarily polymerized.

81

Figure 4.7 Diacetylene functionality incorporated self-registering polymer III

SH

0
0)? CM” C000 H—N——> N\/—Sj<\rN\/_S:<\n
HO

OH

3 III
Scheme 4.6 Synthesis of a 20C-self-registering polymer incorporating diacetylene

(polymer III)

82

 

O C22H§2COCI O HzN SH H
O \ V 0; >Hj:\O(N\/—_S N\/—_S "
HO HO O HO O
OH

00 O O O

4 IV

Scheme 4.7 Synthesis of a 30C-self—registering polymer incorporating diacetylene
(polymer IV)

After the formation of the polymer, the diacetylene residues can be further
polymerized to form the two dimensional polymer-secured self-registering structure
under UV irradiation (figure 4.8). There are two kinds of films-- free-standing films and
unsupported films prepared with the diacetylene incorporated polymers, which will be

further discussed.

83

 

I ,/ 1, .1/ ‘
/ / I, {‘1’ x
/ ‘-, A , ‘.«~.‘ M
/ [a | I \ «a
I. / ' / /{ I .. ‘ if \ -F
’2’ a, W In, E F” 2 “P it 5
i g n x
I, 4
t
'l
l N on-polar region ,-

 

 

mas—w ...... M 0».

Figure 4.8 Secondary polymerization of organic template formed with diacetylene
incorporated self-registering polymer

4.3.2.1 Preparation of freestanding films made from the diacetylene incorporated
self-registering polymers

Freestanding films made from diacetylene incorporated self-registering polymers were
made into a very concentrated solution. The solution was then cast across a fine metal
wire loop. After the film was air dried, it was put in water overnight to enable the
self-registering process. After the rearrangement, the freestanding film attached to the
metal loop was formed. Further inorganic phase modification was achieved through

dipping the metal loop into the semiconductor solution.

The membrane was further polymerized with irradiation by UV light to crosslink the

diacetylene moieties as shown in figure 4.8. After the diacetylene moieties crosslink, a

84

layer made of alternating double-triple bonds was formed. This system can correspond
to environmental changes (such as molecules combing at the polar region, temperature,
light or current) with signals that can be detected--including color changes. In addition,
photopolymerization of the diacetylene groups further stablizes the system and makes it

more rigid.

4.3.2.2 Preparation of unsupported film made from the diacetylene incorporated
self-registering polymer:

A primarily polymerized diacetylene incorporated self-registering polymer solution
was cast on top of water. After the solvent was evaporated, a water-supported
self-registering polymer membrane was formed at the interface of water and air. To
modify it with inorganic elements, semiconductor metal salts were dissolved in the water
subphase. After overnight incubation, the hierarchical material was subjected to UV
irradiation to crosslink the diacetylene moieties. Fendler and co-workers4 prepared
sufficiently small metal sulfide particles to realize nanosize quantum effects by infusion
of H28 from the air-water interface. This method was also used here to make
semiconductor sulfide nanoparticles. Under treatment of H28 gas, small semiconductor
particles with nanoscale sizes were formed and stabilized on the membrane formed from
the polymer.

The systems we prepared with the diacetylene incorporated self-registering polymers

have inorganic phases with nanosized particles (which can also be called quantum dots)

85

in the polar region separated by the diacetylene groups in the electronically and/or
optically responsive region by the hydrophobic, insulating hydrocarbon chains. This
gives the structure multi-functionalized-layer properties. Once it is coated to the substrate,
the system brings all these unique properties to the surface, and makes it highly

applicable in areas such as device fabrication.

4.4 Experimental Section:

Preparation of compound 1: Hexadecanoyl chloride in 10 ml dichloromethane was put
in a dropping funnel. (S)-(-)-5-(hydroxymethyl)-2(Sm-furanone 2.28 g was dissolved in
40 ml dichloromethane and 5 ml pyridine, and was then mixed and stirred in a 100 ml
round bottom flask in an ice bath. The funnel was connected to the flask and the acid
chloride was added to the mixture drop by drop. After all of the acid chloride was added,
the reaction was incubated for 2 hous in an ice bath and 2 hours at room temperature.
Dichloromethane 50 ml and a hydrochloric acid solution (5 ml HCl in 25 ml ice water)
was added to the product. After washing, the aqueous portion was discarded. The product
was washed again with 30 ml of water, then 30 ml of a sodium bicarbonate solution.
Sodium sulfate was added to the product to absorb excess water. The liquid was drained
into another flask. The solvent was evaporated and compound 1 was achieved. 'H NMR
(300 MHz, CDC13), 7.55 (t, 1H), 6.28 (d, 1H), 5.34 (q, 1H), 4.42 (d, 2H), 2.40 (m, 2H),

1.40 (m, 23H)

86

Preparation of Polymer I: 2-Aminoethyl dihydrogen phosphate 0.5592g and 1.65 ml
TEA was mixed in a flask. Water was added dropwise untill the solid dissolved, and the
reaction proceeded for 5 minutes before removing the water by evaporation. The mixture
was added to compound 1 and 60ml solvent of 1:1 dichloromethane : tetrahydrofurane.
This reaction was incubated then for 2 hours in an ice bath, 2 hours at room temperature
and 4 hours between 50 to 60 °C. The solvent was evaporated and the mixture was heated
at 100 °C for three hours to obtain polymer 1. 'H NMR (300 MHz, CDC13), 5.65 (broad),
5.18, 3.6, 3.18, 2.2, 0.81-1.68 (m). l3C NMR (300 MHz, CDC13), 13.85, 19.64, 22.03,
29.82, 58.95. ”P NMR (300 MHz,CDC13), 2.2. IR (CHClg, NaCl), 3377, 2961, 2926,

2874, 2851, 2166, 1743, 1464, 1381, 1172, 1068, 929, 887, 729.

Synthesis of polymer II: 2-Mercaptoethylamine 0.2669g was dissolved in 10 ml
methanol, and then poured into a dropping funnel. The 2-mercaptoethylamine was added
dropwise to a 100ml round bottom flask placed in an ice bath with 1.2180 g of compound
1 dissolved in 50 ml dichloromethane. The reaction proceeded for 2 hours on an ice bath,
2 hours at 50 °C. After the removal of all the solvent, the mixture was heated at 100 °C
overnight to obtain polymer 11. 'H NMR (300 MHz, coca), 2.39m, 1.53, 1.24, 0.89.
'3C NMR (300 MHz, CDCl3 ), 73.6, 64.2, 58.8, 40.5, 37.4, 34.6, 31.1, 30.8, 29.6, 28.1,

25.2, 18.9. IR (CHC13, NaCl), 3351, 2919, 2851, 1701, 1558, 1541, 1466, 1178, 1068,

87

939

Preparation of compound 3: Docosadiynedionic 476mg acid was mixed with 5 ml
oxalyl chloride in dichloromethane 5 ml, and reacted at 70°C for 10 hours with refluxing.
The excess oxalyl chloride and toluene were removed quickly under low pressure by
rotoevaporation. Toluene 10 ml was added to the product and evaporated immediately to
make mixture 1. (S)-(-)-5- (hydroxymethyl)-2(5H)-furanone 150 mg, 1 ml pyridine and
0.5 g potassium carbonate were mixed in 5 m1 anhydrous dichloromethane to make
mixture 2. Mixture 2 was poured into mixture 1 and reacted at room temperature
overnight. The final product, compound 3, was purified with a silica column. lH NMR
(300 MHz, CDC13), 6.00 (d, 1H), 5.12 (q, 1H), 4.18 (d, 2H), 2.40 (m, 6H), 1.40 (m, 24H).

l3C NMR (300 MHz, CDC13), 19.9, 24.6, 28.4, 33.8, 62.2, 81.7, 123.8, 152.2.

Synthesis of polymer III: 2-Aminoethanethiol 0.023 g was prepared into 10 ml
methanol solution with a 10 ml volumetric flask. The methanol solution 2 ml was
combined with 0.117g compound 3. Tetrahydrofuran 10 ml was added to the mixture and
the reaction proceeded for 5 hrs. The mixture was then heated at 70 °C for another 3
hours with refluxing and polymer III was formed. 1H NMR (300 MHz, CDC13), 2.35,
2.25, 1.68, 1.51, 1.30. ”C NMR (300 MHz, CDC13), 18.1, 24.9, 28.4, 28.6, 28.8, 29.0,

29.1, 34.2, 39.0, 39.3, 39.6, 39.9, 40.2, 40.6, 40.8, 71.8, 83.2.

88

Synthesis of polymer IV: Compound 4 0.1875 g (obtained from Dr Hollingsworth)
was dissolved in dichloromethane. Exactly one equivalent (0.0275 g) of
2-aminoethanethiol was rinsed into dichloromethane solution of compound 4, mixed and
the solvent were evaporated overnight. The mixture was incubated at overnight to obtain
polymer IV. 'H NMR (300 MHz, CDCl3), 2.75 (t), 2.52 (d), 2.18 (m), 1.46 (m), 1.26 (m).
l3C NMR (300 MHz, CDC13), 18.1, 24.9, 28.4, 28.6, 28.8, 29.0, 29.1, 34.2, 39.0, 39.3,
39.6, 39.9, 40.2, 40.6, 40.8, 71.8, 83.2. IR(CHC13, NaCl), 3260, 2910, 2120, 1730, 1400,

860, 780, 620

Preparation of the polyethylene supported membrane made of polymer 1: Polymer 1
was designed as a system to imitate the template control over inorganic minerals in the
biomineralization process. A few drops of water were put into the ethanol solution of
polymer 1 to make it cloudy. Plastic pieces (1 cm x 1 cm) were dipped into the cloudy
solution for one hour to be coated. The polymer coated plastic piece was put into water
for 10 minutes to remove the excess polymer and the polymer that was not attached well
to the plastic substrate. A dialysis bag was filled with water, and the polymer coated
plastic piece was put into it before the open end was sealed. The bags were quickly stirred

in a supersaturated calcium phosphate solution for different lengths of time.

89

Preparation of the unsupported membrane made from polymer [1: Polymer 11,
dissolved in toluene, was deposited on top of the cadmium perchlorate aqueous solution.
After incubation of 1 day, ordered crystals of cadmium perchlorate was formed under the
control of the organic template. The cadmium perchlorate solution was then diluted
several times until sodium sulfide tests showed no precipitation. Sodium selenide was put

into the aqueous solution and the system was incubated again.

Preparation of the freestanding film made from polymer II: Polymer 11 was prepared
as a very concentrated solution. Fine metal wire loops with diameters of 2-3 mm were
dipped into the concentrated solution. A thin membrane was seen formed in the middle of
the loop. After the solvent was evaporated, the metal loops were put in water overnight.
Cadmium perchlorate and zinc sulfate solutions were used separately to modify the
membranes. After one day of incubation, the loops were dipped in water for several
minutes to remove the excess metal ions. The loops were respectively put into sodium

selenide or sodium sulfide solutions to make CdS, CdSe, ZnS and ZnSe nanostructures.

Preparation of the pinhole supported freestanding film made from polymer II:
Polymer 11 10 mg was dissolved in 40 ml ethanol and sonicated. Water 40 ml was added
to the suspension before another portion of ethanol was introduced. The mixture was

sonicated and centrifuged. The upper clear liquid was brushed across the pinhole in a

90

polyethylene sheet that is drilled by needle with diameter of 500 microns. After solvent
was evaporated, the membrane was dipped into water for 1.5 hours. For inorganic phase
incorporation, the plastic piece with the membrane was put into cadmium perchlorate
solution for one hour, and then water to remove the excess cadmium ions. Afterwards, the
membrane was put in a very diluted sodium selenide solution for 6 hours and the

membrane prepared was kept in a watch glass.

Preparation of the freestanding film made from polymer III: The primarily
polymerized polymer III was dissolved in the least amount of pyridine to form a very
concentrated solution. The fine metal wire loop was dipped in the solution of the polymer
to form a polymer membrane attached to the loop. The film was air dried, and then put in
water overnight. Further inorganic phase modification was achieved by dipping the metal
loop into a semiconductor cation salt solution (such as cadmium perchlorate or zinc
sulfate). After one day of incubation, the loops were dipped in water for several minutes
to get rid of the excess salts or ions. The loops were subsequently put into sodium
selenide or sodium sulfide solutions respectively to make CdS, CdSe, ZnS and ZnSe
nanostructures. The membrane was further polymerized with irradiation of UV light to

crosslink the diacetylene moieties.

Preparation of the unsupported film made from polymer III: Drops of ethyl acetate

91

solution of the primarily polymerized polymer III were added on top of water. After the
ethyl acetate evaporated, the water-supported self-registering polymer membrane was
formed at the interface of water and air. To modify the membrane with inorganic
elements, cadmium perchlorate hydrate or zinc sulfate was dissolved in the water
subphase to form the cation on the organic template. After incubation overnight, the
hierarchical material was subjected to UV irradiation for 20 minutes. Aluminum stubs
were used to scrape the organic membrane with metal ions chelated from the interface.
The aluminum stubs were then put in a bottle containing a mixture of sodium sulfide and

sulfuric acid to rqalace the ions with sulfide.

92

BIBLIOGRAPHY

1 a) Jansen, J. F. G. A.; Feringa, B. L. Tetrahedron Lett. 1989, 30, 5481; b) Jansen, J. F. (1
A.; Feringa, B. L. Tetrahedron Lett. 1991, 32, 3239; c) Van Oeveren, A.; Jansen, J. F. G

A.; Feringa, B. L. J. Org. Chem. 1994, 59, 5999.
2 http://biomaterials.bme.northwestem.edu/Papers/encyclopediapdf

3 Dekany, I.; Nagy, L.; Turi, L.; Kiraly, Z.; Kotov, N. A.; Fendler, J. H., Langmuir; 1996,
12(15), 3709-3715

4 Yu, J, etc, J. app]. polymer sci, 2004, 92, 749-756

93

CHAPTER 5
EVALUATION AND CHARACTERIZATION OF THE SYSTEMS

ABSTRACT

The three self-registering polymeric systems were evaluated with a variety of methods
including laser scanning confocal micrOSCOpy, x-ray diffraction and electron scanning
microscopy. In the phospholipid membrane mimetics (polymer 1) system, the
modification of a plastic film and the subsequent deposition of calcium phosphate was
confirmed by staining with a cationic dye and the dark field mode of scanning laser
confocal microscopy respectively. In the self-registering polymeric system with long
saturated fatty acid chains and sulfur functions in the polar region (polymer II) system,
lamellar sheet were found to be the dominant morphology. The system displays
fluorescence due to the nanoparticles deposited on the membrane surface. In addition, rod
shape single crystals were found in the pores distributed on the surface of these lamellar
sheets. In self-registering polymeric system modified with polydiacetylene functions in
the long fatty acid chain and sulfur functions in the polar region (polymer 111) system,
lamellar sheets again were found to be the dominating morphology. The fluorescence

behavior of the polymer system is greatly modified by different nanoparticle depositions.

94

In this chapter, we used a variety of methods such as laser scanning confocal
microscopy, atomic force microscopy, x-ray diffraction and electronic scanning
microscopy to evaluate the self-registering polymer systems that we developed. Three
systems were developed: a phospholipid membrane mimetic (polymer 1), a
self-registering polymeric system with long saturated fatty acid chains and sulfur
functions in the polar region (polymer II) and a self-registering polymeric system
modified with polydiacetylene functions in the long fatty acid chains and sulfur functions
in the polar region (polymer III). Polymer I was developed as a supported (substrate
deposited) 2-dimensional membrane system. For polymer II and polymer 111, two kinds
of membranes were prepared for each. An unsupported membrane system which was
formed at the interface of an aqueous phase and an immiscible organic phase was one
membrane system. The membrane can be lifted from the interface and applied to different
substrates or supports for further characterizations or for use in various applications. The
other system studied was a self-standing edge-supported membrane. This was formed
inside of a circular metal wire loop or a pinhole in a polyethylene sheet. These systems
were then subjected to further modifications such as the incorporation of inorganic

phases.

95

5.1 Evaluation of surface coverage of a polyethylene substrate by polymer 1 and
characterization of the modified surface after calcium phosphate deposition
Verification of modification of polyethylene by staining with a cationic dye

Modification of the polyethylene film with polymer I should lead to the formation of an
array of phosphate groups on its surface similar to the array formed on the surface of a
cell. The cationic dye crystal violet (structure shown in figure 5.1) interacts strongly with
cell membranes and should therefore stain the polyethylene film if it was successfully
coated. The comparison between a sample of an uncoated polyethylene film and a coated
film after staining with crystal violet was shown in Figure 5.2. The coated film is stained
much more purple by crystal violet than the uncoated one. This result confirms that the
surface was successfirlly modified and should be capable of functioning as a template or

foundation for the deposition of calcium phosphate.
\Nlé
O /
\N O N

Figure 5.1 Structure of cationic dye crystal violet

96

 

Figure 5.2 Comparison of bare polyethylene piece (left) and polyethylene piece modified
with polymer I (right) after crystal violet treatment. Notice that the polymer coated piece
is stained more purple by crystal violet

Study of calcium phosphate deposition on the polymer I modified polyethylene

The calcium phosphate deposition on the polymer I modified polyethylene was studied
by optical laser scanning confocal microscopy in dark field mode. In dark field
microscopy, transparent areas will appear to be dark and highly refractile areas (6. g. those
modified by salt deposition) will appear to be bright. This mode of light microscopy is
therefore an excellent one for evaluating the deposition of highly refractive inorganic
materials on a transparent organic matrix. In figure 5.3A, the polymer I modified
polyethylene, which was kept in the presence of calcium and phosphate ions for one
week, shows early stages of mineral deposition and partial coverage (i.e. more dark
regions). While in figure 5.38 a similar substrate after five weeks of calcification shows
complete coverage (i.e. more dark regions). The deposition difference can also be seen by
the naked eye (figure 5.4). The plastic piece on the left doesn’t have any coverage,
therefore is very transparent. The polymer I modified polyethylene is much more opaque

due to the polymer and its inorganic phase coverage.

97

 

A B

Figure 5.3 Dark field optics micrographs of polymer modified polyethylene calcification
A: one week of deposition with partial inorganic phase coverage; B: 5 weeks of
deposition with complete inorganic phase coverage. Notice that there are more darker
areas (i.e. no salt deposition) in A the substrate area in B is very bright, indicating
complete salt deposition over the whole surface

 

Figure 5.4 Comparison between a polymer I modified surface and a bare ethylene surface.
Left: transparent unmodified ethylene surface; Right: opaque polymer I modified surface

5.2 Fluorescence and morphology study of unsupported membrane prepared with
self- registering polymer [1 and the observation of rod shape CdSe single crystals
\Vrth cadmium selenide modification, different morphologies such as lamellar phases

sheets, macroporous hexagonal networks (beehive architecture), vesicle spheres, and

98

combinations of these were all observed by laser scanning confocal microscopy under
fluorescence optics (figure 5.5). Among these possible morphologies, the lamellar

morphology dominated.

 

Figure 5.5 Fluorescence micrograph of cadmium selenide modified unsupported
membrane made from polymer H (A) macroporous hexagonal networks (B) lamellar
phases (C) combination of macroporous hexagonal networks and lamellar phases (D)
vesicles phases

In the macroporous morphology, hexagonally shaped pores are packed closely in
arrays with diameters of 10 to 20 microns. The most likely arrangement is one in which
the amphiphilic molecules form double layer structures with the polar heads facing
towards the open areas of the pores, which are highly hydrated, and the hydrophobic tails

are within the double layer. This is essentially the same as the hexagonal inverted phase

99

CDo)

the amphiphilic molecules form double layer structures with the polar heads facing
towards the open areas of the pores, which are highly hydrated, and the hydrophobic tails
are within the double layer. This is essentially the same as the hexagonal inverted phase
observed in biomembranes. The macroporous morphology has an intermediate stability. It
is a mesophase and much more unstable than the lamellar phase because the hydrophobic
tails are still partially exposed to water.

The vesicular phase is nominally the most stable of the three because no edges are
exposed. However it is kinetically unfavored, therefore in some systems it might not be
as common as the lamellar morphology.

Lamellar phases were by far the most common morphology observed. The size of the
folded sheets was often 1 mm in length and 500 microns in width. \Vrth higher
magnification, small pores with diameters of about 200 nm could be seen on the surface
of the sheets. Rod shape particles were observed lying on the surface of the sheets. The
size, shape and morphology of these particles indicate that these are microstructured
CdSe single crystals which were formed using these porous templates as “molds”.

Using a porous template as the matrix to produce a high aspect ratio microstructured
material has been widely studied because it is one of the most efficient and facile
methods'. Anoidic alumina oxide and nuclear track etch polymer membranes are two
commonly used templatesz. One interesting work regarding the synthesis of

Metal-CdSe-Metal nanowires is illustrated in figure 5.63. The preparation process of

cf) 0 l

Au-CdSe-Au is shown on the left side of figure 5.6. A polycarbonate template was
coated on one side with Ag (A). The template was then filled with approximately 1
micron of Ag (B). An Au segment was deposited (C), followed by the CdSe layer (D) and
another Au layer (E). In the end, the Ag backing and the membrane were dissolved and
the free-standing nanowires were formed (F). The right side of figure 5.6 shows the SEM

images of 350-nm diameter Au-CdSe—Au nanowires.

 

Figure 5.6 Template growth and release of Au—CdSe-Au nanowires (left) and SEM images of
Au-CdSe-Au nanowires (right) (Adapted from reference 3)

The unsupported membrane system formed from our polymer II as templates provided
size and crystal structure control over the microstructure of CdSe to yield a
monodispersed, single crystal micro-rod array by a similar mechanism. When the
unsupported polymer 11 membrane was overlayed on cadmium perchlorate solution,
cadmium ions diffused into the pores of the lamellar sheets and were trapped inside. The
pores protected the cadmium ions through the washing process afterwards. When

selenide ions were introduced into the system, they combined with the cadmium ions to

101

 

”CF20

 

nucleate crystallites that even finally grew into uniform sized single crystals whose
dimensions were controlled by the dimensions of these pores.

When this sample was observed under a laser scanning confocal microscope, local
heating by power dissipation of the laser induced phase transitions in the membrane,
including melting, and the CdSe single crystal was released In figure 5.7, the free CdSe
single crystals with widths of 2 microns and lengths of 20 microns can be observed lying
on the amorphous membrane surface. The widths corresponds to the diameter of the
pores and the lengths to the thickness of the polymer II template.

In figure 8a, pores that contain the cadmium selenide single crystals appear to be very
bright under dark field optics because of the high refractility of inorganic crystals. They
have diameters of about 2 microns, which agree with the size of the CdSe single crystals
observed. Some empty pores with the same size can also be seen. In other areas, such as
shown in figure 8b, the distribution of pore-template CdSe single crystals is more
compact.

Some smaller sized rods were observed under phase-contrast optics as shown in figure
5.9. These rods are 500 nm in width and 5 microns in length, and the occurrence is fairly
rare. None of the rods were fluorescent because they are not of the correct length scale (a
few nanometers) in any dimension to display this property. However, as seen earlier
(figure 5.6), the surface of the membrane evidently contained nanoparticles of the

appropriate size to display fluorescence.

102

Here we observed the formation of monodispersed rod-shape CdSe single crystals
with assistant from porous membrane templates. These high aspect ratio semiconducting
materials hold promise for a wide range of potential applications, including chemistry,

physics, electronics, optics, material science, and biomedical sciences.

v

1.20 pm

 

Figure 5.7 Single crystals of cadmium selenide under dark field optics (Left: CdSe single
crystals dispersed on an amorphous subsuate; Right: one common CdSe single crystal
with length of 20 um and width of 2pm)

Pore filled wih CdSe
" single crystal

‘.

  

gle crystal (Left: the pores in a lamellar
membrane with growing CdSe single crystals and without; Right: compact dispersion of
CdSe single crystals)

103

     

Figure 5.9 Phase contrast micrograph of smaller sized rods dispersed on porous
membrane

5.3 Fluorescence study of a self-standing edge-supported membrane prepared with
the self-registering polymer II
Self-standing edge-supported membranes were prepared by two similar methods. The
first method involved dipping the self-registering polymer 11 membrane attached to a
metal loop in water overnight. The membrane was treated with the appropriate metal salt
solution and then with the desired anion. We called this edge-supported self-standing
membrane. The other method involved painting the polymer 11 solution across a pinhole
with a 500 micron diameter in a polyethylene sheet. We called the membrane prepared
this way pinhole supported self-standing membranes.
Lamellar sheet phases are the main morphology observed in self-standing
edge-supported membranes made from self-registering polymer II. In the pinhole
supported self-standing membrane modified with CdSe, a flat fine sheet with

fluorescence across the entire visible light range was observed under fluorescence optics

(figure 10). Red, green and blue fluorescence was observed at excitation wavelengths
of 543 nm, 488 nm and 633 nm, respectively. The blue fluorescence was probably due to
frequency doubling phenomena, which was evidence of a distribution of different sizes of
nanoparticles. The red, green, blue fluorescence opens up possible applications of this
material for ultra-thin color displays based on photostable (non-bleachable) fluorescence

and blue laser development.

 

Figure 5.10 Fluorescence micrograph of cadmium selenide modified pinhole supported membrane
made from polymer 11. Notice that the membrane is a fine flat sheet displaying fluorescence across the
entire visible spectrum.

105

FCP).

C

 

 

 

Figure 5.11 Fluorescence micrograph of a free-standing membrane prepared with
polymer II (A: dark unmodified polymer 11 membrane; B: ZnS modified polymer 11
membrane with pink/purple fluorescence; C: CdS modified polymer 11 membrane with
blue/green fluorescence). The membrane is not laying flat on the substrate and the figures
show the top of a ridge or undulation that is in the focal plane.

Winn:
._4

1—4
"1 Illll

Sl'lllll
.—'<

D—t
\ll lllll

r——4
“Hum

 

Figure 5.12 Fluorescence micrograph of self standing membrane prepared with polymer
11 modified with CdS (upper row) and ZnS (lower row), both display fluorescence across
the entire visible spectrum

106

Before deposition of the inorganic phases, the edge-supported self-standing polymer 11
membrane showed weak fluorescence (figure 5.11A). Deposition of the different
inorganic systems onto the polymer membranes led to a display of different fluorescent
properties. The CdS modified membrane displayed blue/ green fluorescence (figure 5. l l B)
and the ZnS modified membrane displayed pink/purple fluorescence (figure 5.11C).
These systems also displayed fluorescence across the entire visible spectrum (figure

5.12).

5.4 Optical property and High long range order of system 111
Optical properties of membrane prepared with polymer III

The membrane prepared from polymer III exhibited the typical light and
solvatochromatic properties for polydiacetylene polymers (figure 5. 13). The blue
membrane was colorless when placed on a glass slide as a thin film and it turned blue on
irradiation with UV light at 233 nm and 365 nm for half an hour. The blue color turned
red on contact with organic solvents such as chloroform and pyridine or longer time of

UV light irradiation (figure 5.14).

107

 

Figure 5.13 The membrane prepared from Polymer 111 (left) becomes blue after UV
excitation (right) due to the photopolymerization of diacetylene units to form an extended
conjugated system

 

Figure 5.14 Chromatic alteration of polydiacetylene after UV irradiation (left: after 0.5 hr,
right: after 6 hrs)

High long range order of diacetylene containing self-registering polymer

X-ray diffraction experiments were carried out on a layer of polymer IV (which is
an analog of polymer III), in order to characterize the degree of order and to quantitate
aspects of the organization of the system (figure 5.15). The X-ray powder diffraction
patterns were sharp and intense, and contained higher order reflections indicating a high

degree of order in the polymer systems. There was a diffraction peak at 2 9 corresponding

108

to 45.1 angstroms. Molecular models indicate that this is equivalent to the height of a
monolayer of the 2-D system. The strongest peak appears at the angle corresponding to
27.1 angstroms. This represents the distance between the tip of the hydrocarbon chain and
the diacetylene groups. The neighboring peak corresponding to 13.4 angstroms
corresponded to the distance between the diacetylene groups and the polar region. A
sharp peak in the wide angle region corresponding to the inter-chain separations of the
alkyl groups was observed at 4.4 angstroms with 2-theta at 20.1. This is the characteristic
separation for alkyl chains in closely packed lamellar systems (the L a ). Higher order

reflections at 26.9, 33.9, 40.9 degree for 2-theta in the same series were observed.

 

 

 

 

“07"”? 'ILJJI' ' 'T'T‘TT W’WT"

 

 

Figure 5.15 X-ray spectra for system 111

5.5 Microscopy study of self standing membrane prepared with polymer III

The main morphology of the system was flat lamellar sheets, some of which tended to
curl at the edge probably to reduce the alignment of the dipoles in the chiral headgroups.
An example of this is shown in figure 5.16. The tubular structure formed by curling
displays much brighter longer wavelength fluorescence than the substrate. This could be
attributed to the differences in diacetylene cooperated side chains packing close to and far
away form the twisting region. In regions close to the twist, the chains should be more
disoriented and appear red as was observed in chain disorientation due to heating and

solvent treatment (figure 5.13, 5.14).

0—4
2011111

 

Figure 5.16 Curling structure of self standing membrane prepared with polymer HI

Figure 5.17 shows the lamellar sheets on a macroscopic scale. The surface appears

uniform over most of its area. When it is observed with higher magnification, porous

structures can be observed. The sizes of the pores are around two microns. One possible

110

origin for their formation is through an inversed hexagonal phase transition such as

those observed in biomembranes.

 

Figure 5.17 Micrographs of self-standing membranes prepared from unmodified polymer
III membrane. A: lamellar unmodified polymer III membrane in dark field optics; B:
porous surface of unmodified polymer III membrane

As we mentioned above, lamellar phases are the main morphology in self standing
polymer III membrane and the same membranes modified with CdS, CdSe, ZnSe, ZnS.
Figure 5.18 shows the lamellar structures that have been observed. In the membranes
prepared from polymer 111, large dull green low fluorescence with more intense purple
and much more intense but smaller pink regions were observed. This is probably due to
different local ordering patterns or domain structures. High wavelength fluorescence is
characteristic of higher ordering and conjugation. Typical membrane systems exist in a
form where some regions are highly ordered, some are partially ordered and others are

disordered. The diacetylenic functions in highly ordered regions are readily polymerized

lll

()Wn \.

 

if the chain alignment is favorable. These regions will give rise to much brighter
fluorescence at much longer wavelength than more disordered regions. Further expansion
of the ordered regions by mechanical stress or solvation will lead to a further red-shifting
of the fluorescence.

Fluorescence spectroscopes were used to characterize the modification of the
membrane system by deposition of zinc and cadmium salts. The fluorescence properties
of the modified membranes were completely different to those of the unmodified
membranes. For example, ZnSe modified membranes mainly displayed red fluorescence,

while CdSe and ZnS modified membranes displayed more purple fluorescence.

5.6 Scanning Electronic Microscope (SEM), Atomic Force Microscopy (AF M) and
fluorescence characterization of unsupported polymer HI membrane and the
membranes incorporated with CdS or ZnS

Under polarization microscope, the unsupported film deposited with ZnS shows
maltese cross spatial pattern in figure 5.19. The maltese cross pattern is characteristic of
the smectic C phase (chiral lamellar system) expected for the polymer layers“. It indicates
the parallel-polarization component existence. The parallel-polarization component will
evolve into far-field pattern in the form of maltese cross or optical four leaf clover. In
other words, the system has a highly ordered multilayer structure.

The unsupported self-registering polymer III membrane treated with cadmium sulfide

112

displayed strong fluorescence properties. Against the weak green fluorescence of the
substrate, CdS nanoparticles exhibited purple fluorescence as shown in figure 5.20. Here
polymer 111 forms a patterned lamellar structure in which cadmium sulfide clusters are
incorporated. The membrane treated with ZnS nanoparticles also displayed much
enhanced fluorescence properties. They formed delicate bright fucia petals.

The main morphology of unsupported membranes is the lamellar sheets as shown in
figure 5.21. Again, the purple and red spots in the fluorescence micrographs indicate that
the membrane is not of uniform crystallinity. Polarized light microscopy clearly
demonstrated the presence of ordered chiral crystalline domains. In this technique a
polarizing filter called an analyzer is oriented so as to cancel the polarization of the laser,
thus leading to a situation where no light reaches the detector. If a polarizing substance is
injected between the laser and the analyzer, the plane of polarization would be rotated,
and light would then be seen through the analyzer. If the membranes were achiral, the
plane of rotation of the polarized laser light would not change after going through the
layer, and would have been blocked by a cross polarizer. In this case only a black
background would be observed. Regions of the membrane that can be observed using the
polarized light microscopy are chiral with very ordered arrangements that are different to

the bulk material.

113

 

.4. .
FEE .aiuqlfl {win . r

Figure 5.18 A photo gallery of fluorescence micrographs for lamellar self standing
polymer III membranes and polymer III membranes modified with ZnSe, CdSe, ZnS and
CdS. A and B are unmodified polymer [11 membrane (A: small areas of intense purple
and pink fluorescence were observed against a background with low dull green
fluorescence; B: blown-up for a small area with intense purple fluorescence), C and D
are ZnSe modified membranes (similar with unmodified membranes), E and F are CdSe
modified membranes (mainly display red fluorescence), G and H are ZnS modified
membranes (areas with ptuple fluorescence is greatly enhanced), I and J are CdS
modified membranes (areas with purple fluorescence is greatly enhanced)

114

 

 

 

 

5“ 11111

5011111

5'" 11m

 

115

— F“ I
r .
‘1’11'11 21111111

 

A B C

Figure 5.20 Comparison of unsupported membrane prepared with polymer IH (A) and the
ones that incorporate with CdS (B) and ZnS (C). A: untreated system 111 with small areas
of intense purple and pink fluorescence against a background with low dull green
fluorescence; B: CdS treated system 111 with big area of intense purple fluorescence; C:
ZnS treated system 111 with delicate bright fucia petals

116

 

P31“ ...“.iualiu. a n! ...‘Vldl hr

Figure 5.21 A photo gallery of fluorescence nricrographs for lamellar self-standing
polymer III membranes. A and B are polarized optical images of an unmodified polymer
[11 membrane, indicating the chirality and ordered arrangements of the membrane; C, D
and E are fluorescence micrographs showing that the membrane is not of uniform
crystallinity; F and G are polarized optical images of polymer III membranes modified
with ZnS; H and I are polarized optical images of polymer III membrane modified with
CdS; J and K are fluorescence images of polymer III membrane modified with CdS,
bigger area of purple fluorescence was observed

117

 

 

 

 

5011111 f“ ,1m
’—4

1011111

 

Scanning Electronic Microscope (SEM) characterization of unsupported polymer

III membrane and the membranes incorporated with CdS or ZnS

118

Scanning Electronic Microscope (SEM) characterization of unsupported polymer
[11 membrane and the membranes incorporated with CdS or ZnS
The unsupported polymer 11] membrane and the same membranes incorporating
CdS or ZnS were examined by scanning electronic microscopy without coating. The
sharp images obtained indicate that the unmodified polymer [11 membrane and those
modified with CdS or ZnS were very conductive without the usual coating with carbon or
gold to aid in imaging.
Figure 5.22A shows an image of an unmodified sample of this material showing it to be
a multiple layered porous polymer membrane. The morphology of the polymer
membrane is flat lamellar, but the material was evidently very fragile because most

images showed evidence of damages of the topmost layer.

5011111

 

Figure 5.22 Scanning electronic microscope image of unsupported polymer [11 membrane
A) multiple layered porous polymer lll membrane B) magnified view of top surface
shown in A (x25) showing 1 ll pores.

119

The pieces shown in the picture have widths between 25 microns to 50 microns and
lengths around 40 microns. Several pores could be observed distributed over the surface
of the membrane. The pores had diameters of around 1 micron and are probably due to
local inverted hexagonal phases.

The CdS or ZnS modified polymer III membranes appeared to be very brittle and
showed cracking patterns on the surface as indicated below in figure 5.23. The
comparison between SEM images of unsupported polymer III membrane and the
membranes incorporated with CdS or ZnS indicate that the modification of CdS or ZnS

bestows the polymer III membranes some inorganic characters.

 

Figure 5.23 SEM micrograph of CdS (A) and ZnS (B) treated polymer III membrane with
cracking patterns

SEM micrographs of higher magnification for CdS (figure 5.24a) and ZnS (figure
5.24b) treated polymer III membrane show no sign of inorganic particles that were
resolvable at the highest magnification studied where morphologies of 1 micron could be

readily discemed.

120

 

A B

Figure 5.24 higher magnification of SEM micrograph of CdS (A) and ZnS (B) treated
polymer lII membrane showing no sign of large crystals

Atomic force microscopy (AF M) characterization of unsupported polymer III
membrane and the membranes incorporated with CdS or ZnS

Atomic force microscopy is used to characterize the packing and surface coverage
of polymer III and size, distribution, general shape and morphology of the nanoparticles
formed with the polymer as template. The systems were formed on the surface of flat
silicon wafers.

The untreated polymer membrane has very rough surfaces compared to the CdS or
ZnS treated ones. After the metal ion treatment, the membrane becomes flat, smooth and
thin. When the polymer membrane is coated with metal ions, it starts to get charged. The
repelling effect of the charges distributed on the surface makes the membrane expand to
form the flat, smooth and thin morphology. The charged layers are also more solvable.
This results in thinning and evening of the layer. Figure 5.25 shows comparison

between the membrane treated with CdS and the unmodified silicon wafer surface. The

121

KC

 

highest points on the particles were around 54 nm above the silicon substrate.

 

0.00 ‘

Figure 5.25 The flat, smooth thin surface of the membrane treated with CdS (right) on a
clean silicon wafer surface (left) at 300 nm, the highest point on the particles were around
54 nm above the silicon substrate

Coverage by CdS or ZnS was not very uniform. For example, for ZnS treated
membrane as shown in figure 5.26, in some areas, the particles are isolated. In some other
areas, confluent coverage with layers of ZnS can be observed. Similar phenomenon is
observed in CdS treated membranes as well. The diameters of the nanoparticles vary
item a few nanometers to around a hundred. The small nanoparticals will display
fluorescence. The highest points on the particles against the silicon substrate on a bigger

scale are around two hundred nanometers.

122

Figure 5.26 Different distributions of nanoparticels. Sparsely distributed nanoparticles of
ZnS (A, 300 nm) and CdS (B, 500 nm); confluent coverage of nanoparticles of ZnS (C,
300 nm) and CdS (D, 500 nm); evenly distributed nanoparticles of ZnS (E, 500 nm);
From A to E, three-dimensional images are on the left side, and two-dimensional images
are on the right side; CdS (F) and ZnS (G) modified membranes with highest point at 236
nm and 190 nm respectively observed at 2 micron

123

 

 

 

 

 

341111111)

 

124

37.45 nm

"1‘ mm
A

     

D
12.42 rm
0
| IN 111111
0.00
B
236.96 nm 190.74 nm

 

125

Part of the polymer membranes shows distorted columnar structure under high
magnification with widths around 20 nm (figure 5.27). This may be caused by screw

delocalization of the stacking lipid crystal molecules.

13.29 nm

9011111

     

Figure 5.27 Distorted columnar structure of polymer membrane observed at size 100 nm
shown in three dimensional (right) and two dimensional (left)

5.7 Conclusion
Form the evaluation results, we can conclude that we have successfully developed
three multiple frmctional self-registering polymeric systems with unique properties that
can be applied in a variety of fields.

Phospholipid biomimetic system, referred as system I, has been successfully coated
onto a polyethylene surface as demonstrated by a cationic dye experiment. Further
deposition of calcium phosphate was found to increase with incubation time in calcium
phosphate solution. System I resembles collagen templates in cells and can be developed
into biomimetic material with potential application as bone filler or scaffold of bone

tissue engineering.

126

 

(r:

 

 

System 11 with sulfur function and saturated fatty acid chain was prepared into two
different kinds of membranes: unsupported and edge-support self-standing membranes.
Morphologies such as beehive macroporous phases, vesicle phases and lamellar sheets
were observed. Among them lamellar structure dominates. High aspect ratio CdSe single
crystals with rod shape of 2 x 20 microns were formed.

System 111 has similar design with system 11 except for the incorporated diacetylene
functional group in the long fatty acid chain. Experiments verify the chromatic change of
membranes prepared form polymer 111 under irradiations. The membrane appears to be
blue with UV light irradiation for a couple hours due to the photopolymerization of
diacetylene groups, and becomes red after 4 more hours of UV irradiation or organic
solvent treatment. The polydiacetylene function groups bestow the system with an
electronically and optically responsive region. For system 111, we did X-ray diffraction
experiment which provides strong evidence that the system bears high long-range order.
The modification effects of nanoparticles on morphology of the membranes were
confirmed through scanning electronic microscopy (SEM) and atomic force microscopy
(AFM). The sharp images obtained from SEM experiments also tell that system 111 is
very conductive.

System 11 and system 111 serve as templates for nanoparticles formation. This enables
the nanoparticles to be synthesized with mild conditions. The nanoparticles formed

display fluorescence across visible spectrum. For system 11 and system 111, due to the

127

 

stablization coming from polymerization, the membranes prepared are robust enough to
self support or be applied on different substrates for coating and properties modification
after their formation. This greatly enhances the possibility of practical applications of the

systems that we have developed.

5.8 Experimental Section

Cationic dye staining experiment: A polyethylene film and the polymer I modified
polyethylene film were dipped in a crystal violet solution separately and immediately
taken out. The excess dye solution was then removed by delicate task wipers. Digital

camera was used to record the images. Images in this thesis are presented in color.

Laser scanning confocal light microscopy experiment: These experiments were
performed on a Zeiss 210 instrument with laser wavelengths at 488 nm, 543 nm, 633 nm.
Images were obtained in dark-field, phase contrast, polarization and fluorescence modes.
For the polarizing mode experiments, an analyzing cross-polarizer was placed on the
objective lens. For self-standing film preparation, the film was attached to a clean glass
slide directly, and for unsupported film preparation, polymer membrane was taken from
the interface and deposited on a clean glass slide which was left in a horizontal position at
room temperature for two hours to allow the solvent to evaporate. Images in this thesis

are presented in color.

128

X-ray diffraction experiment: This study was performed on a Rigaku instrument with a
Rotaflex rotating copper anode operating at 45 kV with a current of 100 mA. The X-ray
beam was collimated with a 1/6 slit and the K a line was selected. A pinch of polymer IV
was dissolved in 8 drops of methanol, and the mixture was sonicated for 10 minutes. One
drop of a mixture of methanol and water (2: 1) was added, and the mixture was sonicated
for 10 min again. The polymer IV suspension formed was kept in oven at 70 °C for 15
min, then left at room temperature for half an hour. The slurry was transferred to the glass
slide, and air dried for one day. The solid deposited on the slide was used for the x-ray
measurement. After the measurement was performed, the part under the x-ray beam

turned to blue, the part without x-ray irradiation is still white.

Scanning electronic microscopy experiment: The experiment was performed on a
JEOL (Japan Electron Optics Laboratories) 6400V with a LaB6 emitter (Noran EDS).
The electronic beam intensity was 7 kV. The unsupported membranes prepared from
polymer III and those modified with nanoparticles were mounted on the SEM aluminum
stub probes by peeling the membranes from the surface of aqueous subphases with the

aluminum stubs.

Atomic force microscopy experiment: These analyses were performed using a

Nanoscope III instrument operating in tapping mode. The film was mounted on a clean

129

silicon wafer by peeling the membranes or those modified with nanoparticles from the
surface of aqueous subphases with the clean silicon wafer. Images in this thesis are

presented in color.

130

 

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2 a) Shingubara, S; Okino, O; Sayama, Y; Sakaue, H; Takahagi, T. Jpn, J. Appl. Phys.
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4 Wang, G; Hollingsworth, RI; Adv. Mater, 2000, 12, 12, 871-874

131

 

 

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