Nanotechnology-unit 1.ppt

Introduction
The definition of nanotechnology is based on the
prefix “nano” which is from the Greek word meaning
“dwarf”. In more technical terms, the word “nano” means
10-9,
or one billionth of something. For comparison, a
virus is roughly 100 nm in size. The word nanotechnology
is generally used when referring to things with the size of
0.1 to 100 nm, beyond that limit, is referred to as
microtechnology.

Nanotechnology: Nanotechnology is making and
manipulating materials and structures that are
smaller than 100 nm in size.
Nanobiotechnology: It is the amalgamation of
biotechnology and nanotechnology
It is a technology for the design,
characterization, production and application of
structures, devices and systems by controlling
shape and size at nanometre scale.
Definition and Scope of Nanotechnology

History of Nanotechnology
1959- Richard Feyman: Stated the famous
statement ”There is plenty of room at the
bottom”. Described a process to manipulate
individual atoms and molecules that might be
developed using precise tools to build and operate
another set of smaller molecules to the required scale
1965- Gordon Moore: Predicted that the number of
transistors that could fit in a given area would double
every 18 months for the next ten years (this became a
reality with pentium4 containing about 40,000,000
transistors)
1974- N Taniguchi: defined the term nanotechnology
as “Nanotechnology mainly consists of the processing
of separation consolidation and deformation of
materials by one atom or by one molecule.

1986 – Eric Drexler promoted the
technological significance in depth and
popularised the subject.
1980’s – Nanotech and nanoscience got a
boost with the invent of STM and AFM
History of Nanotechnology contd …

Persons involved in Nanotechnology research
Ralph Merkle: Nanotechnology theorist
Robert Feritas: Nanomedicine theorist
Sumio Lijima: Discoverer of nanotubes
Richard Smalley, Harry Kroto - Discoverer of
Buckminsterfullerene
Gerd Binnig, Heinrich Rohrer- Inventor of scanning
tunneling microscope
Phaedon Avouris- First electronic device made out of
carbon nanotubes
David Tomanek – To understand fundamental properties
of nanostructured materials using advanced numerical
techniques.

Applications of Nanomaterials Contd..

Applications: Security
 Security is a broad field, covering everything from the
security of our borders to the security of our
infrastructure to the security of our computer networks.
 Superior, lightweight materials for armed forces and air
crafts
 Quantum cryptography — cryptography that utilizes the
unique properties of quantum mechanics — will provide
unbreakable security for businesses, government, and
military.
 Chemical sensors based on nanotechnology will be
incredibly sensitive — capable of pinpointing a single
molecule out of billions.
 Nanometals, nano-sized particles of metal such as
nanoaluminum, are more chemically reactive because
of their small size and greater surface area. (Stronger
explosives)

Applications: Healthcare
 The lab-on-a-chip is waiting in the wings to
analyze a patient’s ailments in an instant,
providing point-of care testing and drug
application.
 New contrast agents will float through the
bloodstream, lighting up problems such as
tumours with incredible accuracy
 Nanotechnology will aid in the delivery of just the
right amount of medicine to the exact spots of
the body that need it most. (Novel drugs)

 Materials designed at the molecular level to take
advantage of their small size and novel properties.
 The two main reasons why materials at the nano scale
can have different properties are increased relative
surface area and new quantum effects. (Quantum
mechanics (QM) is a set of scientific principles
describing the known behavior of energy and matter
that predominate at the atomic and subatomic scales).
 The unique properties of these nanomaterials give
them novel electrical, catalytic, magnetic, mechanical,
thermal, or imaging features that are highly desirable
for applications in commercial, medical, military, and
environmental sectors.
Nanomaterials

The most current nanomaterials could be organized
into four types:
i. Carbon Based Materials
ii. Metal Based Materials
iii. Alluminosilicates (Imogolite)
iv. Calcium phosphates (Hydroxyapatite)
v. Dendrimers
vi. Nano carriers

Nanomaterials have unique properties

CARBON BASED
NANOMATERIALS
Composed of carbon,
commonly taking the
form of hollow spheres,
ellipsoids, or tubes.
NANOTUBES
BUCKYBALLS
FULLERENES

METAL BASED
NANOMATERIALS
Include quantum dots,
nanoshells, nano gold
probes and metal
oxides particles.
Changing their size
changes optical
properties.
NANOSHELLS
QUANTUM DOTS
MAGNETIC
NANOPARTICLES
NANOPARTICL
E PROBES

DENDRIMERS
These are globular,
highly branched
nanostructures with
numerous chain ends,
tailored to perform
specific chemical
functions.

NANO CARRIERS
Combine nanoparticles
with other nanoparticles or
with larger, bulk-type
materials to enhance
physical properties.
MICELLES
LIPOSOMES
POLYMER
NANO
PARTICLE

Alluminosilicates nanomaterials (Imogolite)
Some naturally ocurring or synthetic clays such as imogolite are
inorganic nanotubes with mesophore.
Imogolite forms tubular structures.
The external tube diameter of imogolite has been shown to be
approx 2.5nm and the tubes are several micrometers long.
The tubes exist in a high degree of order as self alligned bundles
Applications: In the field of catalysis support, ceramic filter, humidity
contolling building materials
Calcium phosphates nanomaterials (hydroxyapatite):
Hydroxyapatite and other related calcium phosphates have been
studied as implant materials in orthopedics and dentistry because of
their excellent hard and soft attachment, biocompatibility and ease
of formation

Allotropes of Carbon:
Carbon allotrope = The different molecular
configurations, that pure carbon can take.
• Diamond
• Graphite
• Amorphous carbon
• Fullerenes
• Carbon nanotubes
• Aggregated diamond nanorods
• Glassy carbon

THE DISCOVERY
Harry Kroto, Richard Smalley and Robert Curl
attempted to create high-temperature conditions in
the laboratory. They vaporized graphite with a
powerful laser in an atmosphere of helium gas.
Graphite
Helium gas atmosphere

The result …
When they analyzed the resulting carbon
clusters, they found many previously unknown
carbon molecules. These varied in size, but
the most common molecule contained 60
carbon atoms. The structure was extremely
stable and they concluded that only a sphere-
like molecule could achieve this level of
stability.
They worked out that the only geometric
shape that could combine 60 carbon atoms
into a spherical structure was a set of
interlocking hexagons and pentagons
Naming: Buckminster fullerene
, Nobel prize for discovery

WHAT ARE FULLERENES?
 Fullerene are a family of carbon allotropes, in the
form of a hollow sphere, ellipsoid, tube or plane.
 Spherical fullerenes are also called buckyballs,
and cylindrical ones are called carbon nanotubes or
buckytubes.
 The fullerene was discovered in 1985 by Robert
Curl, Harold Kroto and Richard Smalley at the
University of Sussex and Rice University, who
named it after Richard Buckminster Fuller, whose
geodesic domes it resembles.

Buckminster fullerene got its name after Richard
Buckminster Fuller who is the architect of geodesic dome
found in this picture…

STRUCTURAL VARIATIONS ON
FULLERENES
 Buckyball clusters: smallest member is C20
(unsaturated version of dodecahedrane) and the
most common is C60;
 Nanotubes: hollow tubes of very small
dimensions, having single or multiple walls;
potential applications in electronics industry;
 Megatubes: larger in diameter than nanotubes
and prepared with walls of different thickness;
potentially used for the transport of a variety of
molecules of different sizes
 Polymers: chain, two-dimensional and three-
dimensional polymers are formed under high
pressure high temperature conditions

PROPERTIES
Physical structure
 Fullerene cages are about 7-15 Angstroms in
diameter. In atomic terms, they are
enormous, but still they are smaller as
compared to many organic molecules.
 They are large closed-cage carbon molecules
consisting of a number of five membered
rings and six membered rings. In order to
make a closed cage, all fullerene molecules
should have the formula of C20+m, where m
is an integer.
Stability
 They are quite stable, breaking the balls
requires temperatures of over 1000 degree
C. At much lower temperatures, fullerenes
will sublime.

Solubility
Fullerenes are sparingly soluble in many
solvents. Common solvents for the
fullerenes include aromatics, such as
toluene, and others like carbon disulfide.
Superconductivity
Intercalation of alkali-metal atoms in solid
C60 leads to metallic behavior. In 1991, it
was revealed that potassium-doped
fullerenes becomes superconducting at 18K.
Since then, superconductivity has been
reported in fullerene doped with various
metals.
Properties contd …

THE PRODUCTION OF FULLERENES
 The first method of production of fullerenes used laser
vaporization of carbon in an inert atmosphere, but this
produced microscopic amounts of fullerenes. In 1990, a
new type of apparatus using an arc to vaporize
graphite was developed in Germany by Kratschmer and
Huffmann.

•This produces a light condensate called fullerene
soot, which contains a variety of different fullerenes.
•The fullerenes are then extracted by a variety of
different solvents of which toluene is the most widely
used because of its low cost, low boiling point and
relatively large capacity for carrying fullerenes.
•Separation and purification happens by column
chromatography

ENDOHEDRAL FULLERENES
 Endohedral fullerenes are
fullerenes that have additional
atoms, ions, or clusters enclosed
within their inner spheres.
 Fullerene compounds are air
sensitive, the oxygen pulls out the
extra atoms out of the fullerene
lattice. Enclosure of an atom inside
a fullerene gives it added stability.
 Two types of endohedral complexes
exist: endohedral
metallofullerenes and non-
metal doped fullerenes

Metallofullerenes
 When the atom trapped inside the fullerene is a metal, it is called
as metallofullerene.
 Metallofullerenes are characterised by the fact that electrons will
transfer from the metal atom to the fullerene cage and that the
metal atom takes a position off-center in the cage.
 These anionic fullerene cages are very stable molecules and do not
have the reactivity associated with ordinary empty fullerenes.
 They are stable in air up to very high temperatures (600 to
850°C). Common metal atoms include lanthanum, yttrium,
scandium etc.
Non-metal doped fullerenes
When the atom trapped inside is a non metal such as
helium, neon, argon, krypton or xenon, it is called as non metal
doped fullerene. In these compounds no charge transfer of the
atom in the center to the carbon atoms of the cage takes place.
The central atom in these endohedral complexes is located in the
center of the cage.

 Discovered by Professors at Rice University
 It is C-60 structure(12 pentagonal and 20
hexagonal)
 C60 is the smallest fullerene molecule in
which no two pentagons share an edge.
 It is also the most common in terms of
natural occurrence, as it can often be found
in soot.
 The van der Waals diameter of a C60
molecule is about 1 nanometer (nm). The
nucleus to nucleus diameter of a C60
molecule is about 0.7 nm.
Buckyballs
(Sphere-like allotropes of carbon)

In C60, hexagons and
pentagons of carbon link
together in a coordinated
fashion to form a hollow,
geodesic dome
Bucky Ball ….
Desirable properties:
1. They exhibit a hollow cage-like shape
2. They are extremely stable and can
withstand very high temperatures and
pressures.
3. The carbon atoms of Bucky balls can
react with other atoms and molecules,
leaving the stable, spherical structure
intact.
4. If they are compressed and then
released they spring back to their
original shape. And they bounce if
they are hurled against a hard surface
such as steel.
5. Fullerenes are sparingly soluble in
many solvents. Common solvents for
the fullerenes include aromatics, such
as toluene, benzene etc.

Applications
 Fullerenes could be put to work as tiny chemical
sponges, mopping up dangerous chemicals from
injured brain tissue. Buckyballs, made soluble in
water, appear to ‘swallow’ and hold free radicals,
thereby reducing the damage to tissue.
 Buckyballs in miniature circuits: Scientists
compressed the Buckyball by 15 per cent,
improving electrical conductivity by more than
100 times compared to the undisturbed molecule.
A tiny electronic component like this could make
miniature circuits feasible.

More Applications
 Buckyballs behaving as 'molecular ball
bearings' allowing surfaces to glide over one
another.
 Buckyball compounds with added potassium
act as superconductors at very low
temperatures.
 Because of the way they stack, Buckyballs
could act as molecular sieves, trapping
particles of particular sizes while leaving
others unaffected.
 Using Buckyballs to improve resolution of
photocopies. They are 1000 times smaller
than the particles used in conventional
photocopier machines.

Even More Applications
 Buckyballs may be used to deliver medicines
to specific tissues and cells, such as those
that have been attacked by a certain
bacteria, protecting the rest of the body
from the toxic effects.
Administration of armed
Buckyballs into a tissue
through a micro syringe. The
delivery system will be
incorporated with drugs that
combat viral attack.

 Improved Medical Resonance Imaging (MRI)
contrast agents and image enhancers that
exploit the carbon cage of a Buckyball to shield
patients from the radioactive materials inside.
 Due to their extremely resilient nature bucky
balls are debated for use in combat armor.
 Bucky balls can be used as lubricants,
protective coatings.

CARBON NANOTUBES
 Nanotubes are cylindrical
fullerenes. These tubes of carbon
are usually only a few nanometres
wide, but they can range from less
than a micrometer to several
millimeters in length. They often
have closed ends, but can be open-
ended as well.
 Their unique molecular structure
results in extraordinary macroscopic
properties, including high tensile
strength, high electrical
conductivity, high ductility, high
resistance to heat, and relative
chemical inactivity

Carbon nanotubes are long, thin
cylinders of bound carbon atoms,
about 100,000 times thinner than
a human hair, and can be single-
or multi-walled. They have
remarkable electronic and
mechanical properties that depend
on atomic structure and more
precisely on the manner in which the
graphene sheet is wrapped to form a
nanotube (chirality). They can be
either metallic or semiconducting
Boron-nitride nanotubes also show potential for similar applications.
BN nanotube. B atoms are in red, N atoms in blue
Nanotubes ….

 They are the strongest and stiffest materials on earth in terms of
tensile strength and elastic modules respectively
 They are not nearly as strong under compression because of their
hallow structure , they tend to under go buckling when placed
under bending stress.
 Multiwalled nanotubes precisely nested within one another exhibit
a strking telescoping property whereby an inner nanotube core
may slide almost without friction within its outer nanotube shell,
thus creating an atomically perfect linear or rotational bearing.
 They behave as a excellent metallic or semi conductors depending
on the configaration.
 All nanotubes are expected to be very good thermal conductors
along the tube exhibiting a property known as ballistic conduction
but good insulators laterally
 Have no signs of toxicity
Properties

Carbon nanotube with metal-
semiconductor junction
Structure of a multi-walled
nanotube
Nanotubes

Nanotubes classified
• Single-wall carbon nanotubes (SWCNTs)-
These are formed by the rolling of a single
layer of graphite (called a graphene layer)

Multiwall carbon
nanotube(MWCNT)
A multiwall carbon nanotube can
similarly be considered to be a coaxial
assembly of cylinders of SWCNTs.

 The joining of two carbon nanotubes with different electrical
properties form a diode.
 Used as composite fibers in polymers to improve mechanical,
thermal and electrical properties of the bulk product.
 Due to their great mechanical properties of the carbon
nanotube they find their applications in clothes, sports gear
and space elevators.
 In electrical circuits because of their unique dimensions to an
unusual current conduction mechanism they make them ideal
component of electrical circuits.
 As a vessel for drug delivery: Drug dosage to be lowered by
localizing its distribution
 Used as electrodes in batteries and capacitors
 Have applications in a variety of fuel cell components
 Used as superstrong fibers will have applications in in body
armour, transmission line cables, woven fabrics and textiles.
Applications of carbon Tubes

Biomedical applications: Cells have been shown to
grow on bucky tubes appear to have no toxic effect.
The ability to chemically modify the sidewalls of
bucky tubes have been used for vascular stunts and
neural growth and regeneration.
Other Applications

What is a Nanoshell?
Nanoshells are optically tunable nanoparticles
that have a dielectric core and an ultra thin
metallic layer of the order of a few
nanometers as its shell.

Properties of gold nanoshells
• Strong optical absorption and yield a
brilliant red colour.
• The optical response of gold
nanoshells depends on the relative
size of the nanoparticle core and the
thickness of the gold shell.
• By varying the relative core and shell
thicknesses, the color of gold
nanoshells can be varied across a
broad range of the optical spectrum
that spans the visible and the near
infrared spectral regions.
Silica core
Gold
coating

Visual demonstration of tunability of metal
nanoshells
Nanobiotechnology and diseases

Properties of Nanoshells
 Coating of colloidal particles with shells offers
the most simple and versatile way of modifying
their surface chemical, reactive, optical,
magnetic and catalytic properties.
 Functional materials with novel properties can
be synthesized using various combinations of
core-shell material and by varying shell
thickness.

OPTICAL PROPERTY
 Metal Nanoparticles show optical absorption in the visible range
of the electromagentic spectrum and sometimes in the IR
Region.
 Their absorption range is mostly from 300-800 nm.
 Preferred core=Silica, shell=Gold
 As mentioned earlier, the optical response of gold nanoshells
depends on the relative size of the nanoparticle core and the
thicknes of the gold shell.
 By varying the relative core/shell thickness, there can be a
good change in color that spans across the broad visible and IR
spectral regions (Tuning)
 The ability to tune nanoshells to a desired wavelength is critical
to in vivo therapeutic applications.
 Human blood and tissue minimally absorb IR wavelengths of
light enabling us to therefore use an external laser to deliver
light to nanoshells either in a tumor or a wound.

Surface Chemical and
Catalytic Properties
 Core-shell particles offer high surface area and
can be used as efficient catalysts.
 Preferred core= titania, shell= Silica
 Bulk Titania is thermally unstable and loses its
surface area readily.
 But coating a thin layer of some other stable
oxide (such as silica) can greatly improve its
catalytic activity.

 Stability of magnetic materials is important when
studying their magnetic properties.
 To improve the surface characteristics and to protect
them from reacting with various species, they are
coated with inert material.
 Silica is the best choice for such a purpose because it
forms stable dispersions, is not magnetic and does not
interfere with magnetic properties of the core.
 Magnetic materials are susceptible to agglomeration
and show anisotropic interactions.
 A thin coating of silica is the best way to protect them
from agglomeration.
 Magnetic particles when coated with silica are
suspended in medium, isotropic interactions are
observed.
Magnetic Properties

Enhancement of thermal stability
 Depression of melting point in nanoparticles as
compared to bulk is observed. This has been
attributed to large surface tension in the case of
nanoparticles. In order to release this tension, they
melt faster as compared to bulk.
 Encapsulation of these nanoparticles by silica
greatly improves their thermal stability.
 By changing the thickness of the shell, variation in
melting point is observed.
 A 60-70 nm thick coating of silica greatly improves
the thermal stability of gold nanoshells (about 300
degrees higher).
 Coating of silica on such shells is a way of
preserving the identity of individual core particles
because of high temperature stability of silica.

Preparation of nanoshells
Nanoshells are prepared by the following methods :
a) Surface coating of a layer of metal by physical
vapour deposition on solid substrates.
b) Thermal evaporation
c) Chemical reduction
d) Other techniques like photochemical reduction etc.

 Nanoshell sensors for precision chemical analysis
 Nanotechnology researchers at Rice University have
designed a sensor which can be specifically used to
obtain chemical information. Raman spectroscopy is
the method widely used for molecular analysis. It
involves the studying of spectrum of light that an
object emits to decipher which elements are present in
the sample.
Applications

 Scientists have long known that they could boost
the Raman light emissions from a sample by a
million times or more just by placing small metal
particles called colloids next to the sample.
 Using the same principle, Rice’s research team has
developed Surface Enhanced Raman Scattering
(SERS). In this sensor, nanoshells are layered
colloids that consist of a core of non conducting
material covered by a thin metallic shell.
 By varying the shell thickness the electrical and
optical properties of nanoshells can be tuned
precisely.

Therapeutic and drug delivery applications
 By carefully choosing a core-to-shell ratio, it is possible
to design novel nanoshell structures which either
absorb light or scatter it effectively.
 Strong absorbers are used in photothermal therapy,
while efficient scatterers can be used in imaging
applications.
 Core shell (mostly gold nanoshells) particles
conjugated with enzymes and antibodies can be
embedded in a matrix of a polymer.

 These polymers include N-isopropyl-acrylamide and
Acrylamide. These have a melting temperature
slightly above body temperature.
 When such a nanoshell and polymer matrix is
illuminated with resonant wavelength, nanoshells
absorb heat and transfer to the local environment.
 This causes collapse of the polymer network and
hence release of drug.
 In core shell particles based drug delivery systems,
the drug can be either encapsulated or adsorbed on
to the shell surface.

Nano weapons to fight cancer
 Nanoshells invented at Rice University have become an
alternative to chemotherapy by killing ONLY cancerous
cells after injection into the blood stream.
 Gold nanoshells are used for cancer because gold is
biocompatible and no antibodies are produced against
it.
 IR wavelengths are used for cancer treatment as they
penetrate the furthest.
 Nanoshells can be tagged with specific antibodies for
diseased tissues or tumours.

 When these nanoshells are inserted into the body,
they get attached to diseased cells and can be
imaged.
 On locating the tumour, it is irradiated with
resonance wavelength of the specific nanoshells
leading to heating of the tumour and hence
destruction of tumour cells.
 The usual treatments for cancer like chemotherapy
or radiotherapy have various side effects like hair
loss, lack of appetite, diarrhoea etc. The process
of attacking the tumour also leads to the loss of
the nearby healthy cells.
 Nanoshells offer an effective and relatively safer
strategy to cure cancer.

Colorimetry and biosensing
 Colorimetric sensing is monitoring changes in the
colour of the nanoparticles which act as sensors.
 Usually gold nanoparticles are used for this purpose
and polynucleotides, oligonucleotides and DNA have
been detected successfully.
 Single strand of DNA was immobilised on gold
nanoshells and was used for the detection of its
complementary DNA strand.
 The intense ruby-red colour changed to blue upon
detection of the complementary DNA due to
agglomeration.

Immunoassay
 Recently, successful detection of immunoglobulins
using gold nanoshells was achieved in saline, serum
and whole blood.
 This system constitutes a simple immunoassay
capable of detecting as small as nanogram per ml
quantities of various analytes in different media
within 10 minutes.
 When introduced into samples containing the
appropriate antigen, the selective antibody-antigen
interaction causes the gold nanoshells to aggregate
thus shifting the wavelength further into the extreme
IR region of the spectrum.

Dendrimers
 Dendrimers are large and complex molecules
with very well-defined chemical structures. They
are nearly perfect monodisperse (basically
meaning of a consistent size and form)
macromolecules with a regular and highly
branched three-dimensional architecture.
 Dendrimers are produced in an iterative sequence
of reaction steps, in which each additional
iteration leads to a higher generation dendrimer.
 They consist of three major architectural
components: core, branches, and end groups.

 The creation of dendrimers, using
specifically-designed chemical
reactions, is one of the best examples
of controlled hierarchical synthesis,
an approach that allows the 'bottom-
up 'creation of complex systems.
 Each new layer creates a new
'generation', with double the number
of active sites (called end groups)
and approximately double the
molecular weight of the previous
generation.
 One of the most appealing aspects of
technologies based on dendrimers is
that it is relatively easy to control
their size, composition and chemical
reactivity very precisely.

Synthesis of dendrimers
 Two Methods : Divergent and Convergent
DIVERGENT METHODS
 In the divergent method ,dendrimer grows outwards from a
multifunctional core molecule The core molecule reacts with
monomer molecules containing one reactive and two
dormant groups giving the first generation dendrimer. Then
the new periphery of the molecule is activated for reactions
with more monomers. The process is repeated for several
generations and a dendrimer is built layer after layer.
Advantages and Disadvantages
Divergent approach is successful for the production of large
quantities of dendrimers. Problems occur from side
reactions and incomplete reactions of the end groups that
lead to structure defects. To prevent side reactions and to
force reactions to completion large excess of reagents is
required. It causes some difficulties in the purification of
the final product

CONVERGENT METHOD
 The convergent methods were developed as a
response to the weaknesses of the divergent
synthesis. In the convergent approach, the
dendrimer is constructed stepwise, starting from
the end groups and progressing inwards. When
the growing branched polymeric arms, called
dendrons, are large enough, they are attached to
a multifunctional core molecule.
Advantages
The convergent growth method has several
advantages. It is relatively easy to purify the
desired product and the occurrence of defects in
the final structure is minimised.

Examples
 The first synthesized dendrimers were
polyamidoamines (PAMAMs). They are also
known as starburst dendrimers. The term
‘starburst’ is a trademark of the Dow Chemicals
Company. Ammonia is used as the core molecule.
In the presence of methanol it reacts with
methylacrylate and then ethylenediamine is
added.
 At the end of each branch there is a free amino
group that can react with two methyl acrylate
monomers and two ethylenediamine molecules.

Properties
 Size, shape and reactivity are determined by generation and
chemical composition of the core, interior branching and surface
functionalities. Its diameter increases linearly per generation
whereas the number of surface groups increases geometrically.
 Dendrimers form a tightly packed ball in solution. This has a great
impact on their rheological properties.
 Dendrimer solutions have significantly lower viscosity than linear
polymers. When the molecular mass of dendrimers increases, their
intrinsic viscosity goes through a maximum at the fourth
generation and then begins to decline.
 The presence of many chain ends is responsible for high solubility
and miscibility and for high reactivity. Dendrimers’ solubility is
strongly influenced by the nature of surface groups. Dendrimers
terminated in hydrophilic groups are soluble in polar solvents,
while dendrimers having hydrophobic end groups are soluble in
nonpolar solvents.
 Lower generation dendrimers which are large enough to be
spherical but do not form a tightly packed surface, have enormous
surface areas in relation to volume.

Dendrimers have some
unique properties because
of their globular shape
and the presence of
internal cavities. The most
important one is the
possibility to encapsulate
guest molecules in the
macromolecule interior.

Advantages of dendrimers
 They are synthesized as a single molecular entity
having high structural and chemical homogeneity
 They offer precisely controlled macromolecular surface,
with a far lower cost than proteins.
 They have broad applicability to interfere with protein-
protein interactions.
 They can be used to precisely control the
pharmacokinetics of drugs.
 They provide a scaffold for the attachment of multiple
functional elements in precise ratios and positions.

Applications
 Dendrimers have been applied in in vitro diagnostics.
Dade International Inc. (U.S.A.) has introduced a new
method in cardiac testing. Proteins present in a blood
sample bind to immunoglobulins which are fixed by
dendrimers to a sheet of glass. The result shows if there is
any heart muscle damage.
 Dendrimers have been tested in preclinical studies as
contrast agents for magnetic resonance They also improve
visualisation of vascular structures in magnetic resonance
angiography (MRA) of the body
 There are attempts to use dendrimers in the targeted
delivery of drugs and other therapeutic agents. Drug
molecules can be loaded both in the interior of the
dendrimers as well as attached to the surface groups
 Water soluble dendrimers are capable of binding and
solubilising small acidic hydrophobic molecules with
antifungal or antibacterial properties. The bound substrates
may be released upon contact with the target organism.
Such complexes may be considered as potential drug
delivery systems

 Dendrimers can act as carriers, called vectors in
gene therapy. Vectors transfer genes through
the cell membrane into the nucleus
 Dendrimers can function as pumping devices,
concentrating reagents in the cavity and expelling
the products from the cavity.
 Dendrimers are used in molecular electronics for
storage of information
 Used for separation and molecular recognition
processes.
 Adhesives, surface coatings or polymer cross-
linking.
 Scaffolds
 Light-harvesting dendrimers that can perform
some of the early functions of artificial
photosynthesis.

SMALLER. . . smaller . . . smaller. In the
semiconductor industry, this mantra translates
to faster . . . faster . . . faster. The question is,
how small can you go?

 A quantum dot is a semiconductor whose excitons are
confined in all three spatial dimensions. As a
result, they have properties that are between
those of bulk semiconductors and those of
discrete molecules.
They were discovered by Louis E. Brus, who was then
at Bell Labs and is now a chemistry professor at
Columbia University.
The term "Quantum Dot" was coined by Mark Reed,
who was then at Texas Instruments and is now a
professor of applied physics at Yale University.

The small size results in new quantum phenomena
that yield some extraordinary properties.
Material properties change dramatically because
quantum effects arise from the confinement of
electrons and "holes" in the material.
Size changes other material properties such as the
electrical and nonlinear optical properties of a
material, making them very different from those
of the material's bulk form.
If a dot is excited, the smaller the dot, the higher the
energy and intensity of its emitted light.
Hence, these very small, semiconducting quantum
dots are gateways to an enormous array of
possible applications and new technologies.

 As small crystals, they can be mixed in liquid
solutions, making them ideal for fluorescent tagging
in biological applications.
 They perform as security taggant.
 In bead form, they can be blended into ink, making
an excellent anti- counterfeiting pigments
 Can be made into film processing legendary for
applications in photonic switching, optical signal
conditioning and mode locking lasers.
 May serve as homeland security devices, detecting
radiations and helping fight terrorism.
Applications

"Here at the Laboratory," says Lee, "we have made
silicon and germanium quantum dots that emit light
throughout the visible spectrum-from the infrared to
the ultraviolet. What makes our dots unique is that
their luminescence can be tuned to any wavelength
over a broad spectral range and be stable under
ambient conditions“.
Micrograph of pyramid-shaped
quantum dots grown from
indium, gallium, and arsenic.
Each dot is about 20 nanometers
wide and 8 nanometers in height.

Making quantum dots
•There are several ways to confine excitons in semiconductors,
resulting in different methods to produce quantum dots.
•In general, quantum wires, wells and dots are grown by
advanced epitaxial techniques in nanocrystals produced
by chemical methods or by ion implantation, or in
nanodevices made by state-of-the-art lithographic
techniques.

Colloidal synthesis
Colloidal semiconductor nanocrystals are synthesized from
precursor compounds dissolved in solutions, much like
traditional chemical processes.
The synthesis of colloidal quantum dots is based on a three
component system composed of: precursors, organic
surfactants, and solvents.
When heating a reaction medium to a sufficiently high
temperature, the precursors chemically transform into
monomers. Once the monomers reach a high enough super
saturation level, the nanocrystal growth starts with a
nucleation process.
The temperature during the growth process is one of the critical
factors in determining optimal conditions for the
nanocrystal growth.
It must be high enough to allow for rearrangement and annealing
of atoms during the synthesis process while being low
enough to promote crystal growth.

Another critical factor that has to be stringently controlled during
nanocrystal growth is the monomer concentration.
The growth process of nanocrystals can occur in two different
regimes, “focusing” and “defocusing”. At high monomer
concentrations, the critical size (the size where nanocrystals
neither grow nor shrink) is relatively small, resulting in growth
of nearly all particles.
In this regime, smaller particles grow faster than large ones (since
larger crystals need more atoms to grow than small crystals)
resulting in “focusing” of the size distribution to yield nearly
monodisperse particles.

There are colloidal methods to produce many different
semiconductors, including cadmium selenide, cadmium
sulfide, indium arsenide, and indium phosphide.
These quantum dots can contain as few as 100 to 100,000 atoms
within the quantum dot volume, with a diameter of 10 to 50
atoms. This corresponds to about 2 to 10 nanometers, and
at 10 nm in diameter, nearly 3 million quantum dots could
be lined up end to end and fit within the width of a human
thumb.
Large quantities of quantum dots may be synthesized via colloidal
synthesis. Colloidal synthesis is by far the cheapest and has
the advantage of being able to occur at bench top
conditions.
It is acknowledged to be the least toxic of all the different forms of
synthesis.

Lee et al. (2002) reported using genetically engineered M13
bacteriophage viruses to create quantum dot
biocomposite structures.
As a background to this work, it had previously been shown that
genetically engineered viruses can recognize specific
semiconductor surfaces through the method of selection
by combinatorial phage display.
Additionally, it is known that liquid crystalline structures of wild-
type viruses (Fd, M13, and TMV) are adjustable by
controlling the solution concentrations, solution ionic
strength, and the external magnetic field applied to the
solutions.
Consequently, the specific recognition properties of the virus can
be used to organize inorganic nanocrystals, forming
ordered arrays over the length scale defined by liquid
crystal formation.
Viral assembely

Using this information, Lee et al. (2000) were able to create self-
assemble highly oriented, self-supporting films from a
phage and ZnS precursor solution.
This system allowed them to vary both the length of
bacteriophage and the type of inorganic material through
genetic modification and selection.

Optical properties
An immediate optical feature of colloidal quantum dots is their
coloration. While the material which makes up a quantum dot
defines its intrinsic energy signature, the quantum confined size of
the nanocrystal is more significant at energies near the band gap.
Thus quantum dots of the same material, but with different sizes,
can emit light of different colors.
The larger the dot, the redder (lower energy) its
fluorescence spectrum. Conversely, smaller dots emit bluer (higher
energy) light. The coloration is directly related to the energy levels
of the quantum dot. Quantitatively speaking, the band gap energy
that determines the energy (and hence color) of the fluoresced light
is inversely proportional to the square of the size of the quantum
dot.

•Larger quantum dots have more energy levels which are more closely
spaced.
•This allows the quantum dot to absorb photons containing less energy,
i.e. those closer to the red end of the spectrum.
•Recent articles in nanotechnology and other journals have begun to
suggest that the shape of the quantum dot may also be a factor in the
coloration, but as yet not enough information has become available.
•Furthermore it was shown recently that the lifetime of fluorescence is
determined by the size. Larger dots have more closely spaced energy
levels in which the electron-hole pair can be trapped.
•Therefore, electron-hole pairs in larger dots live longer and thus these
large dots show a larger lifetime.
• Similar to a molecule, a quantum dot has both a quantized energy
spectrum and a quantized density of electronic states near the band edge.

Applications
In modern biological analysis, various kinds of organic dyes are
used.
However, with each passing year, more flexibility is being required
of these dyes, and the traditional dyes are often unable to
meet the expectations.
To this end, quantum dots have quickly filled in the role, being
found to be superior to traditional organic dyes on several
counts, one of the most immediately obvious being
brightness (owing to the high quantum yield) as well as
their stability (much less photo destruction).
Drawback: For single-particle tracking, the irregular blinking of
quantum dots is a minor drawback.
The use of quantum dots for highly sensitive cellular imaging has
seen major advances over the past decade.
The improved photostability of quantum dots for example, allows
the acquisition of many consecutive focal-plane images
that can be reconstructed into a high-resolution three-
dimensional image.

Another application that takes advantage of the extraordinary
photostability of quantum dot probes is the real-time tracking of
molecules and cells over extended periods of time.
 Researchers were able to observe quantum dots in lymph nodes of
mice for more than 4 months.
Semiconductor quantum dots have also been employed for in vitro
imaging of pre-labeled cells.
The ability to image single-cell migration in real time is expected to
be important to several research areas such as
embryogenesis, cancer metastasis, stem-cell therapeutics,
and lymphocyte immunology.
Scientists have proven that quantum dots are dramatically better
than existing methods for delivering a gene-silencing tool,
known as siRNA, into cells.


•First attempts have been made in using quantum dots for tumor
targeting under in vivo conditions. There exist two basic targeting
schemes: active targeting and passive targeting.
•In the case of active targeting, quantum dots are functionalized with
tumor specific binding sites to specifically bind to tumor cells.
•Passive targeting utilizes enhanced permeation and retention of tumor
cells for the delivery of quantum dot probes.
•Fast growing tumor cells typically have more permeable membranes
than healthy cells, allowing the leakage of small nanoparticles into the
cell body.
•Moreover, tumor cells lack an effective lymphatic drainage system,
which leads to subsequent nanoparticle accumulation.

•One of the remaining issues with quantum dot probes is their in vivo
toxicity.
•CdSe nanocrystals for example are highly toxic to cultured cells
under UV illumination. The energy of UV irradiation is close to
the covalent chemical bond energy of CdSe nanocrystals.
•As a result, semiconductor particles can be dissolved, in a process
known as photolysis, to release toxic cadmium ions into the
culture medium.
•In the absence of UV irradiation, however, quantum dots with a
stable polymer coating have been found to be essentially
nontoxic. Then again, only little is known about the excretion
process of polymer-protected quantum dots from living
organisms. These and other questions must be carefully
examined before quantum dot applications in tumor or
vascular imaging can be approved for human clinical use.
•Another cutting edge application of quantum dots is also being
researched as potential inorganic fluorophore for intra-
operative detection of tumors using fluorescence spectroscopy.

WHAT IS NANOFABRICATION?
 Nanofabrication is the design and manufacture of
devices with dimensions measured in nanometers,
essentially dealing with dimensions less than
100nm.
 Nanofabrication is of interest to computer engineers
because it opens the door to super-high-density
microprocessors and memory chips.
 Nanofabrication has caught the attention of the
medical industry with regard to drug delivery
systems, nanosurgery using nanorobotic devices
etc.
 It is also being extensively researched for use in
military and aerospace applications.

Classic Approach to
fabrication:
• Top down approach –
Nanostructures are made by
stripping layer by layer from
the top. An example involves
scaling down integrated-circuit
( IC ) fabrication, i.e., by
removing one atom at a time
until the desired structure
emerges.
• Bottom up approach – This
relies on self assembly process
where nanostructures are built
atom by atom from the
bottom. An example involves
the assembly of a chip atom-
by-atom; this would resemble
bricklaying.

Types of nanofabrication
 Nanolithography refers to the fabrication of
nanometer-scale structures by patterning
substrates with at least one lateral dimension
between the size of an individual atom and 100 nm
by employing interaction of beams of photons.
It results in the selective removal or deposition
of material onto a substrate in a pre determined
pattern.
 Self assembly is a bottom up process in which
components arrange themselves into structured
units or patterns from a base.

 Any form of lithography essentially means patterning or
printing on a smooth surface by exploiting chemical
interactions to obtain images or characters. In case of
nanolithography, many different principles are used for
creating nanometer scale structures.
 The basic types:
 Optical/Photolithography
 Electron beam lithography
 Ion beam lithography
 Extreme Ultraviolet (EUV) Lithography
 X-Ray Lithography
Nanolithography

Alternate Lithography techniques:
Micro contact printing
Nano imprint lithography
Scanned probe lithography
Dip pen lithography

Nano lithography (contd.)
Optical lithography
It has been the predominant pattern technique since the
advent of the semi conductor age which is capable of producing
some 100nm patterns with the use of very short wavelengths
(currently 248-365nm).
X-Ray lithography
X-ray lithography can be extended to an optical resolution of
0.8nm by using the short wavelength of 1 nanometer for the
illumination.
Electron beam direct write lithography
The use of a beam of electrons to produce a pattern typically
in a polymeric resist PMMA{Polymethyl(methacrylate)}
Extreme Ultraviolet lithography
It is a form of lithography using ultra short wavelengths
(13.5nm).

Optical Lithography:
 It is a process used in
nanofabrication to selectively
remove parts of a thin film (or the
bulk of a substrate).
 It uses light to transfer a
geometric pattern from a
photomask to a light-sensitive
chemical (‘photoresist’, or simply
‘resist’) on the substrate.
 A series of chemical treatments
then engraves the exposure
pattern into the material
underneath the photoresist.

The basic procedure involves:
Cleaning and preparation of the wafer
Exposure and developing the photoresist coated wafer
Etching/Thin film deposition/lift off
Photoresist removal
Photolithography normally employs light from sources
like gas discharge lamps with mercury and mixture of
noble gases like xenon etc. More recently deep UV
(<300nm) produced from lasers are being used.

Electron Beam Lithography :
 It is the practice of using a beam of electrons to generate patterns
on a surface covered with a resist.
 The primary advantage of electron beam lithography is that it is
one of the ways to beat the diffraction limit of light.
 It uses a focused beam of electrons to form the circuit patterns
needed for material deposition on (or removal from) the wafer.
It does not use a mask, electron beams are used to directly etch on
the wafer surface.
 It offers higher patterning resolution than optical lithography
because of the shorter wavelength possessed by the 10-50 keV
electrons that it employs.

Ion Beam Lithography
 This is a variation of the electron beam lithography technique,
using an focused ion beam (FIB) instead of an electron beam, which
scans across the substrate surface and exposes the sensitive
coating.
 A grid of pixels is superimposed on the substrate surface, each
pixel having a unique address. The pattern data is transferred to the
controlling computer, which then directs the electron beam to realize
the pattern on the substrate pixel by pixel.
 The advantages of ion beam lithography include
• computer controlled beam
• minimized back scattering
• no necessity for mask
• minimized diffraction effects
• higher resolution
• accurate surface feature registration
• ion sensitive resists are better responsive

Extreme Ultraviolet
lithography:
 It uses 13.5nm EUV wavelength
 All matters absorbs EUV therefore
procedure to be carried out in vacuum
 All optical instruments and photo resists
are made of multilayer defect free
mirrors which reflect light by means of
interlayer interference

X Ray Lithography:
 The short wavelengths of 0.8 nm X-rays
overcome diffraction limits in the resolution of the
otherwise competent optical lithography.
 Deep X-ray lithography uses yet shorter
wavelengths, about 0.1 nm with modified
procedures, to fabricate deeper structures

The mask consists of an X-ray absorber, typically of gold
or compounds of tantalum or tungsten, on a membrane
that is transparent to X-rays, typically of silicon carbide or
diamond
The figure is an example of a quantum dot array
generated by three dimensional x ray lithography:

Refers to a family of techniques for fabricating or replicating
structures using "elastomeric stamps, molds, and conformable
photomasks" It is called "soft" because it uses elastomeric
materials most notably PDMS (Polydimethylsiloxane). Soft
lithography is generally used to construct features measured on
the micrometer to nanometer scale.
Soft lithography includes the technologies of Micro Contact
Printing (µCP), Replica moulding (REM), Micro transfer
moulding (µTM), Micro moulding in capillaries (MIMIC) and
Solvent Assisted microcontact moulding (SAMIM) (From Xia et
al.) Patterning by etching at the nanoscale (PENs)
Soft Lithography

One of the soft lithography procedures,
Micro contact printing as discussed by Xia and Whitesides, is as
follows:
The lithography procedures (photolithography, EBL, etc.) are
followed to etch a desired pattern onto a substrate (usually silicon)
Next, the stamp is created by pouring a degassed resin overtop of
the etched wafer. Common resins include PDMS and Flurosilicon.
Removing the cured resin from the substrate, a stamp contoured to
your pattern is acquired.
 The stamp is then "inked" by placing it in a bath of inking solution
(for example, in ethanol) and ODT (octadecanethiol) for a short
period of time(Figure 1). The ink molecules then diffuse into the
stamp (Figure 2).
The inked stamp is brought in contact with the substrate for a
certain length of time,allowing ink molecules to transfer onto the
substrate surface. The stamp is removed, leaving the desired single-
molecule thick pattern on the substrate.
Steps 4 and 5 are repeated for each substrate on which the pattern
is desired

Figure 2 - ODT from the solution settles
down onto the PDMS stamp. Stamp now
has ODT attached to it which acts as the
ink.
Figure 1 - "Inking" a stamp. PDMS
stamp with pattern is placed in
Ethanol and ODT solution
Figure 3 - The PDMS stamp with
the ODT is placed on the gold
substrate. When the stamp is
removed, the ODT in contact with
the gold stays stuck to the gold.
Thus the pattern from the stamp is
transferred to the gold via the ODT
"ink."

Types of soft lithography
 Micromoulding in capillaries (MIMIC):1um
 Microtransfer moulding:250nm
 Solvent Assisted Microcontact
Moulding(SAMIM):60nm
 Replica Moulding
 Microcontact printing:300nm

Is a powerful instrument for imaging surfaces at the atomic level.
Its development in 1981 earned its inventors, gerd binnig and
heinrich rohrer (at IBM zürich), the nobel prize in physics in
1986.
For an STM, good resolution is considered to be 0.1 nm lateral resolution and
0.01 nm depth resolution. With this resolution, individual atoms
within materials are routinely imaged and manipulated.
The STM can be used not only in ultra high vacuum but also in air, water and
various other liquid or gas ambient, and at temperatures ranging
from near zero kelvin's to a few hundred degrees celsius.
The STM is based on the concept of quantum tunneling. When a conducting t
ip is brought very near to the surface to be examined, a bias (voltage
difference) applied between the two can allow electrons to tunnel
through the vacuum between them.
Scanning tunneling microscope (STM):

Scanning Tunneling Microscopy (STM):
In 1981:Direct visualization of surface atoms was made using STM
Tunneling is the process in which electrons can pass from one
metal to another even though they are not in contact. This process
occurs by coupling of electronic states between the two surfaces
A sharp tip is attached to a piezoelectric translator (material that
expands and contracts according to the amount of electric current
that travels through it) can position the tip with angstrom
precision(10-10m)
As the tip is scanned over the surface, electrons move between the
tip and the sample.
By attempting to maintain a constant current using a feedback loop
monitored by a computer the piezoelectric receives a signal from the
computer to raise or lower the tip as it scans over the surface.
Plotting the changes in the tip height and position produces a three
dimensional image surface yielding the ability to view the locations
of single atoms and to manipulate their atomic positions.

Scanning tunneling microscope (STM):

SCANNING TUNNELING MICROSCOPE
(STM):
 The resulting tunneling current is a function of tip
position, applied voltage, and the local density of
states (LDOS) of the sample.
 Information is acquired by monitoring the current as
the tip's position scans across the surface, and is
usually displayed in image form.
 STM can be a challenging technique, as it can
require extremely clean and stable surfaces, sharp
tips, excellent vibration control, and sophisticated
electronics.

The Atomic Force Microscope was developed to overcome a basic
drawback with STM – that it can only image conducting or
semiconducting surfaces. The AFM however has the advantage of
imaging almost any type of surface, including polymers, ceramics,
composites, glass and biological samples.
The atomic force microscope moves a sharp probe over the
specimen surface while keeping the distance between the probe tip
and the surface constant.
It does this by exerting a very small amount of force on the
tip, just enough to maintain a constant distance but not enough force
to damage the surface.
The vertical motion of the tip usually is followed by measuring
the deflection of a laser beam that strikes the lever holding the
probe.
Unlike the scanning tunneling microscope, the atomic force
microscope can be used to study surfaces that do not conduct
electricity well. The atomic force microscope has been used to
study the interactions between the E. coli GroES and GroEL
chaperonin proteins, to map plasmids by locating restriction enzymes
bound to specific sites, and to follow the behavior of living bacteria
and other cells.

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