1

CHAPTER I

INTRODUCTION

1.0 General consideration

Nanotechnology and Nanoparticles are generally considered an invention of

modern science. However they have a long history which can be thousands of years.

The people however did not know what they were doing. In the last twenty years there

has been a massive advance of nanomaterials in material science and nanotechnology.

In this panorama silver and gold nanoparticles (AgNP and AuNP hereafter)

are playing a protagonist role. The reason for AgNP and AuNP success lies in a

favorable combination of physico-chemical properties and advances in chemical

synthesis. The main characteristic of AgNP and AuNP is the Surface Plasmon

Absorption (SPA), which has 105 - 10

6 larger extinction cross sections than ordinary

molecular chromophores and is also more intense than that of other metal particles,

due to the weak coupling to the interband transition. The frequency of silver and gold

SPA can also be tuned from visible to near infrared, acting on the shape, size or

nanoparticle assembly. Since the surface chemistry of AgNP and AuNP is very simple

there has been a significant progress in AgNP and AuNP synthesis with tailored shape

or size and has provided a gamut of tools with engineered properties, opening the

access of nanotechnology to manifold applications. Furthermore AgNP and AuNP

have a high chemical stability, photo stability and especially AuNP are nontoxic for

living organisms. Their physiochemical stability, bright color and biocompatibility

explain why the utilization of these particles is dated back to the 5th

century.

1.1 Historical Background

Solutions of liquid gold have been mentioned by Egyptian and Chinese

authors around 5th

century BC. In fact ancients believed in their metaphysical and

healing powers1. Colloids of silver and gold have also been used in the Ancient

Roman times to color glass with intense shades of yellow, red, or mauve, depending

2

upon the concentration of the two metals. A fine example is the famous Lycurgus

cup2 (Fig. 1.1.1) in the British museum, which is dated 4

th century AD. For all the

middle ages Au colloids have also been used in medicines believing in their curative

properties for various diseases1.

Figure 1.1.1 The Lycurgus cup dates from the fourth century AD. In reflected light

(daylight) the glass appears to be green, but when light is transmitted from the inside

of the vessel it is red (by Courtesy of the British Museum)2.

In the 9th

century the Islamic world produced brightly colored luster porcelain3

using reducing metal oxide or metals in a vinegar solution and heating the dried

genista upto 6000 C. In the 15

th century Italian artisans in Gubbio and Deruta were

also able to produce brightly colored porcelain called luster. The luster contained

silver and silver copper alloy nanoparticles. In the 16th

century, the alchemist

Paracelsus claimed to have created a potion called “Arum Potabile” (Latin, Potable

Gold). In the 17th

century glass coloring process was further refined by Andrew

Cassius and John Kunchel by contriving “Purple of Cassius”1. This is a precipitate of

gold colloid and stannic hydroxide, which they have added to the base glass.

Michael Faraday4 around 1850 was the first to recognize that the red color of

gold colloid was due to the minute size of the gold particles. Graham5 first introduced

the word “colloid” in 1861. It was used to describe the very low sedimentation and

non crystalline state, appearances of aqueous solutions made up of compounds known

to be insoluble in water such as silver and gold. In its starting definition the term

colloid implied the suspension of a phase (solid or liquid) into a second phase and was

used for suspension that neither settled nor deposited spontaneously. These properties

3

have led Graham to postulate that these colloidal particles should be large enough

(above 1 nm) and of relatively weak size in order to settle out (below 1 m).

In 1898 Zsigmondy6 a

prepared the first colloidal gold in diluted solution. The

red color was the same as in the ‘Purple of Cassius”. Zsigmondy invented the ultra

microscope and with the help of this he was able to show that the colloidal gold was

due to the tiny gold particles. This laid the foundation for the modern colloid

chemistry. For this pioneering work, Zsigmondy was awarded the Nobel Prize for

Chemistry in 1925. Apart from Zsigmondy, Svedberg and Mie6 b

were also interested

in understanding synthesis and properties of colloidal gold. The surface plasmon

band was rationalized in a master publication by Mie in 1908.

1.2 Modern Approach

The fundamental work for synthesis of colloidal metal particles was first

developed by Turkevitch8 in 1951. He started a systematic study of AuNP synthesis

by various methods, using Transmission Electron Microscopy (TEM), for optimizing

the preparative conditions. This ultimately led to the commonly called “Turkevitch

Method” for preparation of colloids.

Nobel laureate Physicist, Richard Feynman9 a

in a lecture before the Annual

Meeting of the American Physical Society in December 1951 predicted that “there is

plenty of room at the bottom”. By these keywords, he pointed out that there is lot of

scope for research on materials of very small scale. By small scale he meant the

dimension of particles in the nanometer range. This attracted a large number of

scientists and researchers towards “nano”, which is still feebly discovered.

In 1974, the word, “Nanotechnology” was introduced by Taniguchi9b

, Japan.

In Greek, nanotechnology is derived from “nano” which means dwarf and

“technologies” which means systematic treatment of an art or craft. Nanotechnology

is based on the ability to explore the objects at the nanoscale and manipulate them

wherever possible. It can also be said to be a science and engineering of making

materials, functional structures and devices on the nanometer scale. In scientific terms

4

“Nano” means 10-9

meter, where 1 nm is equivalent to one thousandth of a

micrometer, 1 millionth of a millimeter and 1 billionth of a meter.

Researchers began to understand that there exists a class of materials with a

dimension of 1 to 100 nanometers or a number of atoms ranging from 10 to 100000.

They are thus too large to be considered as molecules, too small for the bulk

materials. Thus the size regime of 1 to 100 nm is the transition between the molecular

and bulk solid state limits. Unlike bulk materials their physicochemical properties

strongly depend upon their size.

Chemistry Nano scale regime Solid State Physics

One atom 10 atom

cluster

100

atom

cluster

1000

atom

cluster

10,000

atom

cluster

1 x !"

atom

cluster

Bulk

0 nm 1 nm 2 nm 3 nm 5 to 7 nm 10 nm More than 100 nm

Thus the emergence of nanoscience is largely attributed to the unique

properties arising from the material dimension that is of the order of electron mean

free path (quantum size effect) and motivated by their diverse applications and

potentialities, unexplored before10

.

Nanomaterials could also exist in the form of clusters as independent separate

particles. Such clusters when brought together coalesce and form bigger particles.

Therefore, it is necessary to keep them in a medium and prevent their fusion. It is thus

an independent particle whose properties are determined by the number of atoms in it.

Clusters are also called Nanoparticles, Quantum dots, Q particles, Coulomb islands,

Mesoscopic systems, or artificial atoms. All these refer to an aggregate of atoms

ranging from a few numbers to few thousand numbers that are bound together.

Nanostructures broadly include Quantum wires, grains, particles, nanotubes,

nonorods, nanofibers, nonofoams, nonocrystals, nonoprisms, self assemblies or thin

films.

Europe has maintained substantial research in solid state chemistry and

physics. It has enjoyed 3 Nobel Prizes within the last 20 years in research related to

5

nanotechnology. The first of this was for the discovery of Quantum Hall Effect,

awarded to Klaus Von Klitzing from the Max Planck Institute, Stuttgart, Germany in

1985. The second is for the discovery of the Scanning Tunneling Microscopy (STM)

awarded to Heinrich Roher and Gerd Binnig at IBM, Zurich, Switzerland. The third

was shared by Jean Marie Lehn, at the College of France in Paris in 1987, along with

two American Chemists (D. J. Cram, ULCA and C. J. Pederson, Dupont) for

stimulation and growth.

The spread of nanotechnology in the last 20 years is strictly due to the

improvement of synthesis techniques and characterization on the nanometer scale.

This has allowed the release of scientist’s innate curiosity towards a field generous of

new physical phenomena and synthetic opportunities. Thus nanotechnology,

nanoscience, nanostructures, nanoparticles and nanoclusters are most widely used

words in scientific literature.

1.3 Quantum Size Effect

The physical properties of nanomaterials are attracting increasing interest,

because they differ significantly from bulk properties as a result of surface or

quantum size effect10

. However, these properties are highly dependent on the size of

the particles and not on interactions between their surface, ligands, contaminants or a

support. This therefore requires the need to control precisely the size distribution of

the particles and their surface state. If a metal with bulk properties is reduced to a size

in the valence band the conduction band decreases respectively to such an extent that

electronic properties change dramatically. When a metal particle size is decreased, the

typical bulk properties like conductivity and magnetism begin to disappear or in other

words a quasi-continuous density of states is replaced by a discrete energy level

structure and band gap increases. The average energy level separation is comparable

to or larger than the experimental energy. The situation in a small molecular cluster is

simple. Three metal atoms for instance form energetically well defined bonding and

antibonding molecular orbitals.

6

Figure 1.3.1 Illustration of the electronic states in (a) a metal particle with bulk

properties and its typical band structure, (b) a larger cluster of cubic close packed

atoms with a small band gap, (c) a simple triatomic cluster with completely separated

bonding and antibonding orbitals.

The quantum size effect arises due to the dramatic reduction in a number of

electrons11

. This occurs most drastically in low band gap materials where the effective

mass of electrons is small. This phenomenon is observed when a colloidal particle

becomes comparable to de Broglie wavelength of charge carrier.

Recently Volokotin12

et. al. clarified that quantum size strongly affects the

thermodynamics of the metal nanoparticles. It has been found from the spectral

studies that the binding energies of metal nanoparticles consisting of fewer atoms than

1000 vary periodically due to quantum size effect. The phenomenon was first reported

by Knight13

et. al. in a experiment, involving sodium nanoparticles. This shows

discontinuous existence of nanoparticles having certain stable structures.

Different methods of organizing quantum dot super lattices of metals and

semiconductors have received significant interest in recent times. The tailoring of

particles sizes and the inter particle separation can in principle cause unique magnetic,

optical and electronic behavior14-18

. For example, several nanocluster assemblies

organized in different length scales have been found to be promising due to their

7

potential application in diverse areas such as opto-electronic devices, single electron

transistors and chemical sensors17-22

.

Recently much attention has been paid to the synthesis and characterization of

bimetallic nanoparticles due to their catalytic, electronic, optical, structural and

thermal properties23-28

and subsequent technological applications as catalyst, sensors,

nanoelectronic devices29-34

and biosensors35

. The properties and applicability of

nanoparticles not only depend upon their size and length but also on the combination

of component metals (composition) and their fine structure as either an alloy or core

shell structures. Bimetallic nanoparticles with well defined alloy structures of noble

metals like Pt-Ru, Pt-Mo, Au-Ag, provide practical examples for the influence of

metals, compositions and their structure on their catalytic properties36-40

. Au-Ag

nanoparticles of alloy type structure exhibit high catalytic activity for low temperature

CO oxidation34,39

and aerobic oxidation of alcohol41

.

1.4 Properties of Metal and Metal oxide Nanoparticles

The dispersions of metal nanoclusters both mono and bimetallic one 13

, can be

easily obtained by the reduction of metal ions in water or organic solvent in the

presence of soluble polymers or ligands. The physical and the chemical properties of

metal nanoparticles depend upon several factors such as the particle size, structure,

the surface, the shape, the organization of the particles and their dispersity. Numerous

studies42-46

on size and shape dependent properties have been carried out in the last

two decades. The size dependent physical and chemical properties have been

reviewed extensively by many scientists47-49

.

a) Mechanical Properties – Mechanical Strength

The clusters interact fully with one another, yet the effect of cluster size is still

important. Clusters of metals and ceramics in the size range of 5 - 25 nm have been

consolidated to form ultrafine grained polycrystals that have mechanical properties

remarkably different from and frequently better than those of their conventional

coarse grained counterparts50

.

8

b) Melting Point

According to theoretical approach the van der Waals forces i.e. cohesive

forces between the atoms and molecules, affect the melting point of the substance.

When clustering of molecules or atoms takes place then depending upon the size of

the cluster the melting point varies. In general it is found that atomization energy per

atom for spherical cluster increases irregularly on going from 1 to 500 atoms.

Since the solid to liquid transitions begin at interfaces, a well known feature of

nanometric particles is the lower melting temperature with respect to the bulk. For

instance gold undergoes a decrease in the melting temperature35

of about 400 0C

going from 20 nm to 5 nm particles and of about 50 0C from bulk to 20 nm particles.

Melting temperature Tm depends on the particle size ‘d’ as,

Tm = A + #

$

%

Following, a general rule for physical properties dependent upon surface to

volume ratio, that can be described by using the size equations structure, as in the

above equation.

c) Colour

The colour of the metal nanoparticles in suspended state changes with the size.

As the size decreases band gap increases and thus the position of the fluorescence

band is shifted to the shorter wave length51

. Since ancient time gold was used to stain

glass with beautiful colours such as red, purple, burgundy52

. Faraday4 attributed this

colour to very finely divided colloidal gold or gold nanoparticles. Gold nanospheres

have a characteristic red colour, while silver nanospheres are yellow. The colour is

due to the collective oscillation of the electrons in the conduction band, known as

surface plasmon oscillation. The oscillation frequency is usually in the visible region

for silver and gold giving rise to the strong surface plasma resonance absorption. This

means that the origin of properties on the nanoscale is different for metal

nanoparticles than for semiconductor nanoparticles.

9

d) Optical Properties

The distinctive colours of colloidal gold and silver are due to a phenomenon

known as plasmon absorbance. Incident light creates oscillations in conduction

electrons on the surface of nanoparticles and electromagnetic radiation is absorbed.

The surface plasmon band (SPB) is due to the collective oscillations of the electrons

at the surface of the nanoparticles, which is correlated with the electromagnetic field

of the incoming light i.e. the excitation of the coherent oscillation of the conduction

band. The study of the SPB has remained an area of very active research from both

scientific and technological standpoints especially when the particles are embedded in

ionic matrices and glasses53, 5 4

. For instance a driving force for this interest is the

application to the photographic process55

. Thus the SPB provides a considerable body

of information on the development of the band structures in metals and has been the

subject of extensive study of optical spectroscopic properties of Au

nanoparticles43,54,56,57

.

According to Mie43

theory the total cross section composed of the SP

absorption and scattering is given as a summation of the overall electric and magnetic

oscillations. The resonances denoted as surface plasmons were described

quantitatively by solving Maxwell’s equation for spherical particles with the

appropriate boundary conditions. Mie’s theory attributes the plasmon band of

spherical particles to the dipole oscillations58

of the free electrons in the conduction

band occupying the energy states immediately above the free electrons in the

conduction band occupying the energy states immediately above Fermi energy level.

The SPB is absent for Au nanoparticles with core diameter less than 2 nm as well as

for bulk gold. The SPB maximum and bandwidth are also influenced by the particle

shape, medium, dielectric constant and temperature. The refractive index59,60

of the

solvent has been shown to induce a shift of the SPB as predicted by Mie theory. The

ligand shell alters the refractive index and causes either a red or blue shift, so that the

spectroscopic data obtained often deviate from the prediction of Mie theory that deals

with naked nanoparticles. The agreement with Mie theory is obtained only when the

shift induced by this ligand shell is taken into account. This shift is especially

significant with thiolate ligands, which are responsible for a strong ligand field

10

interacting with the surface electron cloud. In fact since all Au nanoparticles need

some kind of stabilizing ligands or polymer, the band energy is rarely exact as

predicted by Mie theory. If the shift of this stabilizer is not considered with the

elliptical particles, the SPB is shifted to higher wavelength as the spacing between

particles is reduced, and this shift is well described as an exponential function of the

gap between the two particles. A red shift for a polarization parallel to the long

particle axis and a blue shift for the orthogonal polarization are reported61

and

rationalized by a dipolar interaction mechanism. The optical thickness cloud can be

used as a measure of efficiency. This parameter is critically dependent on the particle

size and refractive index. Therefore impurities can be easily detected62

since the

refractive index of Au nanoparticles greatly differs from that of gold oxide or gold

chloride. Another influential parameter is the core charge60,62,63

. Excess electronic

charge causes shifts to higher energy, whereas electron deficiency causes shift to

lower energy. The SPB width was found to increase with decreasing size in the

intrinsic size region (mean diameter smaller than 25 nm) and also to increase with

increasing size in intrinsic size region (mean diameter larger than 25 nm). A small

temperature effect was also found. It was proposed that the dominant electronic

dephasing mechanism involves electron-electron interactions rather than electron

phonon coupling63

. The SPB position in Au@SiO2 (including films) was accurately

predicted. Thus it is possible to synthesize composite material with optical properties

that lie anywhere between those of transparent glass and those of metallic gold64-66.

A near field optical antenna effect was used to measure the line shape of these

SPB in single Au nanoparticle and the result was found to be in agreement with Mie

theory. Double peak shapes caused by electromagnetic coupling between close lying

particles were observed67

.

For rod like particles68

, the extinction maximum for incident electric fields

polarized along the long axis occurs at a longer wavelength than the !max for

polarization along the particle radius. Selective suppression69,70

of extinction of the

SPB was also observed. The coagulation71

(along with Oswald ripening) of Au

nanoparticles dispersed in organic liquids was dramatically accelerated by visible

11

light, and the process was shown to be wavelength dependent, UV irradiation caused

coalescence.

e) Fluorescence

Fluorescence studies of Au nanoparticles carried out under various

conditions72-74

, including femto second emission73

and steady state investigation of the

interaction between thiolate ligands and the gold core74

. Indeed resonant energy

transfer was observed in fluorescent ligand capped Au nanoparticles. This

phenomenon is of great interest in biophotonics75

and materials science76-78

. Both

radiative and nanoradiative rates critically depend on the size and shape of the Au

nanoparticles, the distance between the dye molecules, the orientation of the dipole

with respect to the dye-nanoparticles axis and the overlap of the molecule’s emission

with the nonaoparticles absorption spectrum79

. Orders of magnitude of higher

efficiencies were obtained with nanometer dimension metal samples80-82

. The

observed increase in the fluorescence83

yield reflected the suppression of the

nonradiative decay upon binding to Au nanoparticles. Visible luminescence has been

reported for water soluble Au nanoparticles, for which a hypothetical mechanism

involving 5 d10

- 6 sp1 interband transition has been suggested

72.

f) Electrochemical Properties

Electrochemical properties of metal nanoparticles are strongly size dependent,

in fact cluster made by few hundred or tens of atoms have been used to investigate the

transition from the metal like capacitive charging to the redox like charging. Upto 15

charging peaks were observed by differential pulse voltametry in case of Au (I).

The electro chemistry of ligand stabilized Au nanoparticles shows the

formation of core charge. Moreover, the quantized capacitance charging of Au

nanoparticles, self assembled monolayers on electrode surfaces could be rectified by

certain hydrophobic electrolyte ions in aqueous solution. Magneto electrochemistry of

Au nanoparticles with quantized capacitance charging shows the influence of

magnetic field on the electro chemistry of Au nanoparticles and in particular, the

effect of electron parity upon their charging states84

. Double layer capacitance was

12

obtained in aqueous media by differential pulse voltametry85

. Altogether,

electrochemistry86-90

has been used in several ways to fabricate and deposit Au

nanoparticles.

g) Electronic Properties

It was found that for, 2 D super lattices consisting of large (5 nm) Au

nanoparticles, the electronic behavior was dominated by the coulomb block effect at

low temperature, while the I-V response was ohmic at room temperature91

. TiO2

nanoparticles exhibited a blue coloration due to stored electrons within the particles

when subjected to UV. A partial disappearance92

of the blue color was seen upon

contact with Au nanoparticles as electrons were transferred from TiO2 to Au

nanoparticles. TiO2 films cast on conducting glass plates were modified, by adsorbing

Au nanoparticles (5 nm diameter) from toluene solution and selective formation of Au

islands and larger particles on the TiO2 surface was observed93

. The effect of spin-

orbit scattering on discrete energy level in Au nanoparticles and other nanoparticles

showed level-to-level fluctuations of the effective g-factor from zeeman splitting and

the statistics were found to be well characterized by random matrix predictions94

.

Energy transfer from a surface bound area near to the gold core was observed upon

photo irradiation95

.

In semiconductor the nanoscale becomes important due to the quantum

confinement of the electrons96

. As the particles size decreased below the Bohr radius

of the semiconductor material used, the electron becomes more confined in the

particle96

. This leads to an increase in the band gap energy. Furthermore the valence

and conduction bands break into quantized energy levels. The study by Bawendi97

, of

the CdSe spherical nanoparticles of various sizes illustrates the macroscopic

realization of quantized energy levels.

h) Chemical Properties

Gold and silver are known for being generally inert and especially gold for not

being attacked by O2 to a significant extent. This makes Au nanoparticles and Ag

nanoparticles stable in ordinary conditions98

. Gold nanoparticles are resistant to strong

oxidizing agents and strong acids however “aqua regia” or solutions containing I- or

13

CN- can immediately dissolve them. Both Au and Ag are reactive with sulphur, in

particular bulk silver often undergoes tanning due to the formation of an Ag2S surface

layer. In case of organic thiols, ligation to nanoparticle surface is particularly effective

for the contemporary presence of sigma type bond, in which sulphur is the electron

density donor and the metal atom is the acceptor, plus a " type bond in which metal

electrons are partially delocalised in molecular orbitals formed between the filled d

orbitals of the metal and the empty d orbitals of sulphur98

.

Solutions of Ag nanoparticles have applications as bactericidal agents because

the Ag+ ions interfere with bacterial metabolism. Since Ag nanoparticles are exposed

to a certain extent surface oxidation by atmospheric O2, sliver sols can release Ag+

ions with concentration sufficient to act as bactericides98

.

The significance of nanoscale quantum confinement of the electrons provides

visualization of the shift in the characteristics of the material depending upon the size

of nanoparticles. The electrons are not as free to move thus creating new samples for

applications hold a promise.

In a metal, electrons are highly delocalized over a large space (i.e. least

confined). As the separation between the valance and conduction bands vanishes it

renders the metal its conducting properties99

, whereas when the separation between

the valences and conduction band becomes comparable to or larger than KT, the metal

becomes semiconductor. When this confinement further increases the energy

separation, the metal becomes insulator. In noble metals the decrease in size below

the electron mean free path gives rise to intensive absorbance in the near UV visible

region.

i) Specific Surface Area

Nanoparticles present a higher surface to volume ratio with decreasing size of

the nanoparticles. Specific surface area is relevant for catalytic reactivity, electrical,

optical and thermal properties, although stabilization of the surface area, nanoparticles

topology (roughness) and interface with support material are also relevant aspects.

When a metal particle decreases in size to become a nanoparticle or a nanocrystal a

14

greater proportion of atoms is found at the surface. The metallic nanoclusters are built

up by hexagonal (HCP) or cubic (CCP) close packed atoms (nuclearities). As this is

the case in most bulk metals, where a central atom is surrounded by 1, 2, 3… closed

packed cells.

Table 1.4.1 Schematic illustration of metal nanocrystals, in closed-shell

configurations with their magic numbers of atoms and the relation between the total

number of atoms and the % of surface atoms100

.

Full Shell

Clusters

Total Numbers

of Atoms

Surface

Atoms (%)

1 shell 11 92

2 shells 55 76

3 shells 147 63

4 shells 309 52

5 shells 561 45

7 shells 1451 35

j) Thermal Properties

As compared to the study of other well known properties of the nanomaterials,

this study has seen slower progress. This is partially due to the difficulties of

experimentally measuring and controlling the thermal transport in nanoscale

dimensions. Atomic Force Microscope101

(AFM) has been introduced to measure the

thermal transport of nanostructures with nanometer scale having a high special

1 Shell 2 Shells

4 Shells 5 Shells7 Shells

3 Shells

15

resolution providing for a promising way to probe the thermal properties with

nanostructures. Recent advances in experiments have showed that certain

nanomaterials have extraordinary thermal properties as compared to their

macroscopic counterparts. This is attributed due to several factors such as the small

size, the special shape; the large interfaces modified the thermal properties of the

nanomaterials rendering them to quite different behavior. As the dimension goes

down to nanoscales, the size of the nanomaterial is comparable to the wavelength and

the mean free path of the photons, so that the phototransfer within the materials will

be changed significantly due to the photon confinement and quantization of photon

transport, resulting in modified thermal properties. For e.g. nanowires102

from silicon

have a much smaller thermal conductivity as compared to bulk silicon. Carbon

nanotubes103

have extreme high thermal conductivity in axial direction leaving high

anisotropy in the heat transports in the materials.

1.5 Synthesis of Metal nanoparticles

Nanoparticles have potentially useful size and shape dependent

properties104-107

. The synthesis of nanoparticles with desired size and shape has

therefore enormous importance, especially in the emerging field of

nanotechnology108-109

. This in fact demands development of a new general methods as

well as improvement over the existing ones. At present the nanoparticles are prepared

by number of physical and chemical methods.

The approaches110

for synthesis or preparation of nanoparticles fall under two

categories. These are “Bottom up” and “Top down”.

a) Bottom Up Synthesis

This involves the building of structures, atom by atom or molecule by

molecule. The formation of structures in this approach is governed by the polarity and

reactivity of the nanoparticle surface111

. This is derived from the fictionalization of

particle surface. The functionalities112

determine the binding capabilities of the

nanoparticles with other molecules, particles and surface. The bottom up approach for

fabrication of nanostructures from stable building blocks has become popular in

current science and engineering. This construction principle mimics biological

16

systems by exploiting the order inducing factors that are imminent to the system113

.

The bottom up methods may in future offer a number of potentially very attractive

advantages. These include experimental simplicity down to the atomic size scale, the

possibility of three dimensional assemblies and the potential for inexpensive mass

fabrication.

Bottom up approaches use single atoms or ions to grow clusters and

nanoparticles wet chemically of rather precise composition, size, and structure. In

order to prevent agglomeration, the nanosized entities have to be protected by

appropriate ligand molecules, serving as a protecting sphere. Thus special conditions

are required for the generation of identical species.

Figure 1.5.1 Bottom up approach for nanoparticle preparation.

b) Top Down Synthesis

This approach of synthesis involves starting with a larger piece of material and

etching, milling or machining a nanostructure from it by removing material (as, for

example in circuits on microchips). This can be done by using techniques such as

precision engineering and lithography, and has been developed and refined over the

last 30 years. Such methods offer reliability and device complexity, although they are

higher in energy usage, and produce more waste than bottom up methods. The

disadvantage of this method is the manipulation of small amounts of atoms at a time

resulting in low fabrication throughput.

17

Figure 1.5.2 Top down approach for nanoparticle preparation.

The various chemical and physical methods available for synthesis or

preparation of metal nanoparticles are discussed as follows.

a) Chemical Reduction Method

It is the most widely used method for nanoparticle synthesis. Metal ion in

aqueous solution is reduced by reducing agents such as hydroquinone, sodium

borohydride, hydrazine, sodium citrate, ascorbic acid etc. to produce nanoparticles.

An electric charge on the reductant particle was found to exert large effect on the

process of nanoparticle formation. The most highly dispersed and stable colloidal

nanoparticles are formed by using anionic reductant114

. All these wet chemical

approaches require the reduction of Ag+ or Au

+ ions and the chemisorption and

physisorption of ligands on the surface of metal nanoparticles to avoid their

coagulation and precipitation.

One of the best known technique is described by Turkivech115,116

, where an

aqueous solution is brought to boil and sodium citrate is used as both as a capping

material and reducing agent. The order of addition of the metal salt and sodium citrate

as well as the temperature, mole ratio of citrate to metal salt and volume of the

solution determined the size of the nanoparticle generated. This synthesis is easy,

quick and reproducible and generates mono dispersed population of nanoparticles

around 20 nm in diameter. This synthesis is widely used to generate metal

nanoparticles for many systems where 20 nm is a desirable diameter, or a seed for

further growth. The use of sodium citrate as a capping material allows easy surface

modification for a variety of applications. Sodium citrate is a weak reducing agent

18

which does not reduce the metal ion at room temperature. Thus the strength of the

reducing is controlled by the temperature of the solution.

A very popular technique of generating small gold nanoparticles has been

reported by Burst117, 118

. The technique is often referred to as creating monolayer

protected clusters, due to the formation of monolayer of thiol molecules on the surface

of the gold nanoparticles. These particles are extremely stable due to the high strength

of the gold–thiol bond. A two phase systems is used where the gold ion is first

transferred to the organic phase by employing a phase transfer agent, typically tetra

octyl ammonium bromide (TOAB). Then sodium borohydride is used to reduce the

gold ions. This reduction is accomplished slowly when the gold ions from the organic

phase come in contact with sodium borohydride from the aqueous phase. Then thiols

can be used to protect the gold ions after they are reduced. This technique is able to

glow clusters around 1.5 nm diameter that are very stable.

Other reducing agents that have been used to synthesis noble metal

nanoparticles are formamide119

, sucrose120

and glucose121

. Other wet chemical

approach exploit reduction of metal salts by organic solvents such as ethanol for gold

and dimethyl formamide for silver or polyols like ethylene glycol in presence of

protecting polymers as polyvinyl pyrrolidone or other stabilizing molecules122

.

b) Gamma ( ) Irradiation Technique

The radiation induced synthesis is one of the most promising strategies

because there are some important advantages to the use of the irradiation123

techniques, compared to the conventional chemical and photochemical methods.

These are :

1) The process is simple and clean.

2) The gamma ray irradiation has harmless future.

3) Controlled reduction of metal ions can be carried out, without using

excess reducing agent or producing undesired oxidation product of the

reductant.

4) The method provides metal nanoparticles in a fully reduced, highly

pure, highly stable state,

5) No disturbing impurities like metal oxides are produced.

19

Radiolytic reduction generally involves radiolysis of aqueous solutions that

provides an efficient method to reduce metal ions and form homo and hetero nuclear

clusters of metals. In the radiolytic method aqueous solutions are exposed to gamma

rays creating solvated electrons, e-aq. These solvated electrons in turn, reduce the

metal ions and the metal atoms eventually coalesce to form aggregates as depicted by

following reactions124

,

&'( radiation

e-aq

, H3O

+, H*, H2

, OH*, H2O2

e-aq

, + M

m+ M

(m – 1)+

e-aq + M

m+ M

0

nM0 M2 … Mn… Magg

The gamma irradiation based strategies have been used to synthesize

gold125, 126

and silver127

nanoparticles. Silver nitrate or hydrogen tetra chlorate aurate

was dissolved in water with 2 propanol and PVP to form primary solution with

predetermined concentration of the metal salts. The mixture solution was bubbled

with pure nitrogen for about 20 minutes to remove oxygen and then irradiated in field

of 60

Co, # ray source at certain irradiation dose rates. Dark brownish silver colloids or

reddish gold colloids were prepared. The addition of PVP128

and small amount of 2-

propanol did not lead to any thermal reduction of the Ag or Au salt. This technique

has also been used to prepare bimetallic alloy metal clusters of Ag-Pd129

.

c) Ultrasound Technique (Sonochemical reduction)

Ultrasonic irradiation treatment is done by means of high density ultrasonic

probe consisting of multiwave ultrasonic generator and barium titanate oscillator. This

technique has been proved useful for preparation of novel materials130

. The chemical

effect of ultrasound cavitation, that is formation, growth and implosive collapse of

bubbles in liquid produces unusual chemical environments131

.

Radical species are generated from water molecules due to the absorption of

ultrasound.

&'( altrasound radiation

H* OH* (sonolysis)

M 2+

+ 2H* M0 + 2H

+

nM0 (M

0)n (aggregates)

20

The OH*radical, produces hydrogen peroxide which oxidizes the metallic

cluster. In the presence of hydrogen the OH* radicals can be scavenged out. In this

method stable fine particles are produced in the presence of the protecting agent such

as surfactant or water soluble polymer.

Silver nanoparticles with a size of 5 nm have been prepared by this

technique132

. An argon / hydrogen environment produced more H* radicals and

prevented the oxidation of silver nanoparticles. The period of sonication was 90

minutes. Gold nanoparticles have also been synthesized by this method133-135

.

d) Laser Ablation Method

Wet chemical methods to prepare metal nanoparticles are based on the

reduction of suitable metal salts in the presence of stabilizers. Laser ablation136

on the

other hand is an alternative route to the chemical synthesis for the preparation of small

metal particles. The main advantage of laser ablation is the high purity of the

nanoparticles wherein pure bulk material and solvents can be used in the preparation.

Another advantage of this method includes its simplicity and versatility with respect

to metals or solvents in the absence of chemical reagents or ions in the final solution.

Furthermore it is shown that laser ablation137

of gold and silver produces stable

colloids.

Metal nanoparticles are produced by focusing Q switched Nd YAG laser

(1064 nm, 8 nsec. pulses) as the radiation source, on a metal wire. The laser used has

an energy output of 5 – 15 mJ. operated at 50 Hz. A 1064 laser beam is focused by a

lens with 50 cm focal length on the metal wire suspended in solvent. Typical ablation

time was 10 minutes.

e) Microwave Heating

Recently microwave irradiation as a heating method has been used in the

synthesis of nanoparticles. In principle this is a high temperature synthesis method.

The generation of nanoparticles originates from the heating effect, rather than the

energy of quantum of microwave. Polar molecules can be heated quickly under the

microwave irradiation. In comparison with conventional heating the microwave

synthesis is quite fast, simple, very energy efficient and also achieves uniform

21

nucleation in shorter crystallization time. In a microwave heating process one

precursor can be heated at much higher heating rates and reach a higher temperature

then its surrounding. A quick simple and energy efficient method based on

microwave138, 139

heating has been developed and is widely used in synthesis of solid

materials. Silver nanoparticles have been synthesized using the non conventional

reducing agent, N,N diphenyl benzamidine and the microwave irradiation method140

.

Different size silver nanoparticles of interesting colors were synthesized.

f) Photochemical Synthesis

Recently a radiolytical and chemical size controlled, with improved mono

dispersity via seed mediated growth of colloidal nanoparticles has been reported125

.

They have followed iterative growth method, i.e. particles were grown in the

immediately previous steps, were used as seeds in the next growth step. Here particles

of various size ranging from an average diameter of 5 –20 nm could be obtained

within a few minutes by UV irradiation at room temperature, in the presence of air.

Furthermore larger size particles (25 to 100 nm) were produced directly from the

original seed particles by varying the ratio of original [seed] to [Au (III)] and using

ascorbic acid as reductant. It should be noted that ascorbate ion is frequently used as a

reducing agent for reduction Au (III) or Ag (I) ions. In a variation of this method pH

dependent condition large spherical gold nanoparticles were synthesized directly from

Au (III) ions141

. There are other reports for synthesis by photochemical methods for

silver142,143

and gold144

nanoparticles.

g) Sol Gel Technique

Traditionally Sol-Gel processing refers to the hydrolysis and condensation of

alkoxide based precursor such as tetra ethyl orthosilicate. Disregarding the nature of

the precursor the Sol- Gel can be characterized by a series of distinct steps145

.

1) Formation of stable solution of alkoxide or solvated metal precursor (the Sol).

2) Gelation resulting from the formation of an oxide or alcohol bridged network

(the gel) by poly condensation or poly esterification that results in a dramatic

increase in the viscosity of the solution.

22

3) Ageing of the gel (syneresis) during which the poly condensation reactions

continue until the gel is transformed into a solid mass, accompanied by

contraction of the gel network and expulsion of the solvent from the gel pores.

4) Drying of the gel when water and other volatile liquids are removed from the

network. This process is complicated due to fundamental changes in the

structure of the gel.

5) Dehydration during which surface bound M–OH groups are removed, there by

stabilizing the gel against rehydration. This is achieved by calcining at

temperature of 800 0C.

6) Densification and decomposition of the gel at high temperature (T greater than

800 0C). The pores of the gel network collapse and remaining organic species

are volatilized. This step is normally reserved for preparation of dense

ceramics and glares.

The Sol-Gel processing of materials coupled with the inherent advantages of

the organically modified silicates make these matrices very attractive as supports and

stabilizers for nanometalic particles in both liquid and solid phases. The

nanobimetallic combinations145-147

such as Au-Pd, Au-Ag, Au-Pt and Ag-Pt have been

synthesized by this method.

h) Microemulsion Synthesis

The preparation of metal nanoparticles using the microemulsion system, where

ionic and non ionic surfactants are used has been well reported148

. Microemulsions are

colloidal “nanodispersions” of water in oil (W/O) or oil in water (O/W) stabilized by

surfactant film. In case of water in oil microemulsions the size of the water droplet is

varied by changing only one parameter W0 (W0 = Water to Surfactant molar ratio).

The size of the nanoparticles is controlled by the size of the droplets of the micro

emulsion.

A reverse micelle system149

has been used for the preparation of silver, gold

and silver gold bimetallic alloy nanoparticles. In the preparation non ionic surfactant,

Triton X 100 and cyclohexane as bulk phase and 1 hexanol as a co surfactant have

been used. The metallic and bimetallic nanoparticles are synthesized at room

temperature through the reduction of metal salts, using sodium borohydride as a

23

reducing agent. The nanoparticles synthesized by this technique are stable for about 6

months. Two reverse micelle system are prepared at room temperature, one contained

reducing agent and the other contained metal salt solutions, as a water domain in the

reverse micelle. Then the two reverse micelles are mixed with continuous stirring. A

change in the color occurred almost immediately after mixing the two reverse

micelles. The size of the nanoparticles was in the range of 5 to 50 nm. Using this

technique silver150,151

and gold152

nanoparticles have been prepared.

i) Templated Synthesis

The synthesis of nanoparticles on templates has generated an increasing

amount of attention in the past few years. Techniques on heterogeneous nucleation in

which seed crystals serve as nucleation site for further deposition and growth of

crystallites can, essentially be considered one of the simpler forms of a template

synthesis. This technique153,154

can be used to increase the average particle size of

nanoparticles such as aqueous Au (III) is deposited on colloidal Au or Ag+ is

deposited on colloidal Ag. This method can be also used for the synthesis of core shell

and onion structure such as deposition Pd, Bi, Sn, or In on colloidal Au155,156

. To

maintain the narrow size distribution of the nanoparticles care must be taken to ensure

that small particles do not nucleate from the solution during the deposition process.

Several new techniques have recently emerged to prevent this occurrence. Brown157

et. al. used hydroxyl amine for the seed mediated growth of colloidal Au, increasing

the average diameter from 12 to 50 nm. The use of hydroxylamine is critical to this

process.

j) Electrochemical Reduction method

Elucidating the electronic structure of metallic nanoparticles is attractive to

scientist. The development of preparative method to obtain enough amounts of the

monodispersed metallic nanoparticles is essential for the observation of different

analytical tool. Many researchers have developed the synthetic methods of

monodispersed metal nanoparticles in the presence of linear polymer158,159

,

ligands160,161

and so on. In general the control of nanoparticles size and shape

constitutes a preparative challenge. Some methodologies have been demonstrated by

24

chemical reduction methods either by adjusting the water contents at a certain

interconnected microemulsion phases for the production of rod like nanoparticles or

though using capping polymers in the preparation of cubic tetrahedral Pt.

nanoparticles162

.

Of all the methodologies developed for the production of metal nanoparticles

the electrochemical methods offers an alternative simple means within reverse

micelles in organic solvent systems. We now successfully apply it to the preparation

Ag and Au, mono and bimetallic nanoclusters within a suitable mixture of organic

solvents and have developed a unique synthetic route in preparing sufficient yield of

these nanoparticles. Our synthetic approach is to control the growth and particle size

by introducing size control reagent into the electrochemical systems in which

appropriate surfactants or ligands are employed. There act is a supporting electrolyte

and a stabilizer for the resulting nanoparticles. Other parameter for controlling the

size of the nanoparticels is current density in electrochemical reduction method.

Pure and size selective nanoparticles can be prepared by electrochemical

reduction method. The process makes the use of an expensive two electrode set up for

25 to 50 ml electrolyte solution. During the synthesis the bulk metal anode is oxidised

and converted into metal cations163

. These cations migrate to the cathode and the

reduction takes place with the formation of the metal atom in zero oxidation state.

The noble metal cluster in the 1 to 30 nm range has been prepared by this

method for fundamental and practical reason. They have unused electronic properties

and used in catalysis, photocatalysis and electrocatalysis (fuel cell technology). In

order to prevent agglomeration of metal particles in solution, synthesis is done in

presence of stabilizer such as ligand (phenanthroline), polymers (PVP) or surfactants

such as tetra alkyl ammonium salts.

The particle size of the nanostructured metal clusters can be controlled by

varying parameter such as types of ligands, by varying of surfactants, their

concentration, temperature, solvent and current density in the electrochemical

synthesis. “True control of particle size remains the most attractive goal for the

synthetic chemist”. The first report of stabilization of metal colloids by tetra alkyl

25

ammonium salt, Pt / acetyl trimethyl ammonium chloride is due to Gratzel164

in 1979.

Thereafter the research in the area of ammonium salt stabilized metal clusters was

intensified by Toshima165

et al. Recently Bollemen166

synthesized

stabilized metal cluster by reduction of metal salt with tri alkyl boron hydride.

Quaternary ammonium compounds are a group of ammonium salt in which organic

groups are substituted for all the four hydrogen of the original ammonium cation.

They have central nitrogen atom which is joined to four organic groups. The organic

group is mainly alkyl or aryl and the nitrogen can be a part of the ring system. They

show a variety of physical, chemical and biological properties and most compounds

are soluble in water and act as strong electrolyte.

In electrochemical synthesis, the choice of the tetra alkyl ammonium

surfactant is governed by the following constraints,

1. It should be adsorbed the metal surface to stabilize the synthesized

nanoparticles.

2. It should be highly soluble and dissociated in the solvent to play the role of a

electrolyte.

3. Hydrophylic- Liophilic balance of the chosen surfactant should be sufficiently

high to prevent any precipitation due to the presence of water, even in the

presence of small quantities in the medium.

The surfactant that statistics the above constraints, such as tetra ethyl

ammonium bromide (TEAB), tetra propyl ammonium bromide (TPAB), tetra butyl

ammonium (TBAB), tetra octyl ammonium bromide (TOAB) are used in the present

work and their structures are shown in the figure.

A controlled current electrolysis is used throughout the synthesis process.

Solvent selection is made (either single or mixture), depending on the solubility of the

ligand or surfactant. The basic principle is simple, viz, electrochemical reduction of

metal ion in the presence of surfactants is served as an electrolyte and as stabilizer.

The salient feature of the process includes high yield, absence of any side products,

easy isolation of the nanoparticles, variation and choice of ligands, and control of

26

particle size by controlling current density. The metal aggregates synthesized do not

coalesce during the process.

1.6 Applications of nanoparticles

Metal nanoparticles have many unique properties. These include large surface

to volume ratio, high surface reaction activity, high catalytic efficiency and strong

adsorptions ability. Thus they have many possible applications in areas such as non-

linear optical switching167

, immunoassay labeling168

and Raman spectroscopy169

enhancement. Thus the nanoparticles do not have only exceptional scientific

attractions but have increasing practical relevance in numerous disciplines like

Physics, Chemistry, Biology, Medicine and Material Science. The nanoparticles are

classified as two types i.e. Engineered or Non- Engineered. Engineered nanoparticles

are intentionally designed and created with physical properties tailored to meet the

need of specific application. They can be end product in themselves, as in the case of

quantum dots or pharmaceutical drugs, sensors for special purpose, or they can be

later incorporated, such as carbon black in rubber products.

a) Catalysis

Catalysis drives many reactions, with the ability to lower the activation energy

of the reaction, and thus increases the rate of reaction and the yield. The use of

nanoparticles as catalyst has increased as nanoparticles properties and reactions are

better understood. The possibility of using less material and have different properties

for different shapes of nanoparticles is very attractive. Nanoparticles catalysis has

been investigated170

for both homogenous and heterogeneous systems. Small clusters

are also found to be catalytically active171,172

, even for materials that display very

limited reactivity on the bulk scale. For example bulk gold is considered as a noble

metal and is unreactive in the bulk state. However, small clusters of gold have been

found to be catalytically active. Many possible explanations have been proposed for

the difference in activity between clusters and bulk gold. They include the electronic

and chemical properties of nanoparticles or the shape, size and oxidation state of the

nanoparticles. The surface support171

is also suggested to be responsible for the

27

catalytic activity. The crystal structure of gold has also been proposed to be important

in the catalytic property of gold.

The Au nanoparticles supported on Co3O4, Fe2O3 or TiO2 are highly active

catalyst under high dispersion for CO and H2 oxidation173,174

, NO reduction175

, water

gas shift reaction176

, CO2 hydrogenation177

and catalytic combustion of methanol178

.

Bimetallic nanoparticles are also applied to hydrogenation catalysts. PVP

stabilized Au–Pd179

bimetallic nanoparticles with various compositions are used as

catalysts for the selective hydrogenation of 1, 3 cyclooctodiene to cycloctane. In these

bimetallic nanoparticles, with palladium content of 80% showed the highest activity

which is greater than Pd nanoparticles themselves.

The greatest attraction of gold nanoparticles catalysis is their green

credentials. They offer the possibility of catalytic oxidations, that use atmospheric air

as the oxidant and forms pure water as the only by product. Recently the aerobic

oxidation of methanol to industrially important chemical, methylformate using an

unsupported nano-porus gold catalyst has been reported178

. The porus catalyst was

prepared by removing silver from gold-silver alloy, a process called dealloying.

b) Nanofluids

Nanofluids are engineered by dispersing nanometer size solid particles in

traditional heat transfer fluids to increase thermal conductivity and heat transfer

performance. The primary function of nanofluid is to dramatically enhance thermal

conductivity in conventional fluids. Since thermal conductivity of solid nanoparticles

is much higher than that of fluids the suspended particles are expected to increase the

thermal conductivity and heat transfer performance. In fact many factors (such as

shape, size, volume, fraction, thermal conductivity of nanoparticles, viscosity as well

as membrane of liquid molecules around the nanoparticles etc.) may influence the

thermal conductivity of nanofluids.

Experimental result should that the enhanced thermal conductivity ratio

increases with the increase of volume fraction of Al2O3 nanoparticles in

nanosuspensions180

. The results of Zhang181

showed that the effective thermal

28

conductivity and thermal diffusivity of Au / toulene, Al2O3 / water and CNT / water

nanofluids at different temp increases with the increase in the particle concentration,

the nanoparticle thermal conductivity and the ratio of the length to the diameter of

CNT. Another important factor that may influence the thermal conductivity of

nanofluids is the liquid solid interface. Liquid molecules close to the liquid solid

surface are known to form a layered structure. A liquid in contact with a solid

interface is more ordered than a bulk liquid. Since crystalline solids (which are

obviously ordered) display much better thermal transport than liquids, such a liquid

layering is expected to lead to a higher thermal conductivity182

.

c) Cosmetics

ZnO nanoparticles183

have received considerable attention due to their unique

properties. They are well known as UV blocking materials especially of light in the

UV region and as such are used widely in cosmetics, paints and fibers. But their high

catalytic activity in oxidation and photochemical reactions restrict their application as

UV blocking materials. Particle surface modification is regarded as an effective way

to restrain the ultrafine particles, from high oxidative and photochemical activity.

d) Industrial Applications

Silver is amongst the widely used metal in the world. Its metallic properties

make it a great conductor. Its antimicrobial property has also been exploited in several

applications. Reducing silver to nonosized particles helps to make the element highly

effective, making it more in demand for several uses in the industry of medicine and

technology. Currently there are many uses of silver nanoparticles in industries that

have resulted in a boost in its demand and production. Silver nanoparticles can be

used as an ingredient for water purifiers and even as an ingredient for inks used in

inkjets. A potential for AgNP industrial use is making it as coating on polyurethane

foams. The material is ideal as it can be washed, dried and stored for long period of

time and still retained same number of nanoparticles. The material is also tested as an

element for water filtration to eliminate bacteria. It is also possible to use it in the air

filtration, air quality management and industrial settings, as well as antibacterial

packing semiconductor nanoparticles.

29

e) Semiconductor nanoparticles

A decrease in the size of semi-conductor nanoparticles results in the shift of

the conduction and valence bands towards more negative and more positive potentials

respectively. It seems to be tempting to use the possibility of controlling the redox

properties by changing their size, in photocatalytic systems, water decomposition,

synthesis and destruction of organic compounds.

The small size of semi conductor nanoparticles can provide high efficiency of

detrapping of light generated electrons and holds the increasing probability of the

photocatalytic process on the surface of the conductor. The idea of using

nanoparticles for photocatalytic water decomposition was first proposed by

Henglein184

and Duonghong185

who demonstrated the possibility of photoredution of

methylviologen in colloidal solutions of semi-conductors. The photo reduction of

methyl viologen on the surface of nanoparticles was studied by laser flashed

photolysis, in colloidal solutions of TiO2185,186

and metal chalcogenides187

. When Cd-

Se is electrochemically deposited188

on electrically conducting glass, agglomerated Q

particles are formed and not a compact film. Such electrodes in photochemical cells

exhibit photocurrent arising from the Q particles. In order to absorb significant

quantities of the incident light 10-20 coatings of Q particles are deposited one over the

other, and thus the released electrons must be transported over many particles to the

back contact. Energy is lost with every jump of the electrons from one particle to

another, so the photocurrent and voltage attainable with such systems are relatively

low. This problem can be overcomed by depositing the Q particles on a porus TiO2

electrode, to which the electron are transferred after light absorption189

. Less than a

monolayer of particles is necessary for practically all the incident light to be absorbed.

f) Solar Cells

Electrochemical studies have demonstrated the electron storing properties of

gold nanoparticles and their ability to act as an electric relay on a nanotemplate

structure190-192

. Beneficial role of gold nanoparticles in photocurrent generation has

been reported by Kuwahara193

in tris 2, 8 bipyridilede ruthenium (II) vilogogen linked

thiol.

30

g) Nanodiagnostic

Molecular recognition is fundamental for the development of clinical

diagnostic tools and therapeutic modalities. Various organic molecules possessing

unique properties have been used to achieve the recognition of different targets (194).

The possibility of combining the ease of handling DNA based modification with

different modification strategies of nanomaterials has showed its applicability195

in

spectroscopy, electrochemistry, magnetism (imaging, purification and detecting and

others). The incorporation of nanomaterials (e.g. AuNP) facilitates signal

transduction, as the signal of recognition can be amplified by several orders of

magnitude, making recognition more effective. They can be modified according the

function of the designated DNA probe and make applications of functional DNA

more practical for molecular recognition in medical diagnostics by taking advantage

of the unusual interaction between the nanomaterials and the living system196

. This

increase in sensitivity and flexibility present numerous advantages197

over more

traditional procedure like fluorescence and chemi-luminescence.

The first report of DNA hybridization event with thiol-DNA modified gold

nanopaticle (Au nanoprobes) was made by Storhoff198

and co-workers and since then

several new approaches and applications has been reported.

h) Drug Delivery

In recent years nanocapsusles and nanospheres surrounded by a polymer

matrix into which the drug is physically and uniformly dispersed are used as drug

delivery devices. These have the ability to circulate for a prolonged period of time,

target a particular organ, as carriers199

of DNA in gene therapy and have the ability to

deliver proteins, particles and genes.

The advantages of using nanoparticles as delivery systems includes,

1) Particle size and surface characteristics can be easily manipulated to achieve

passive and active drug targeting after parental administration.

2) They control and sustain release of the drug during transportation and at the

site of localization, altering organic distribution of the drug and subsequent

31

clearance of the drug so as to achieve increase in drug efficacy and avoid side

effects.

3) Drug loading is relatively high and drugs can be incorporated into the system

without any chemical reaction thus preserving drug activity.

4) Site specific targeting can be achieved by attaching targeting ligands to the

surface of the particles or use of magnetic guidance.

5) The system can be used for various routes of administration including oral,

parental, intra-ocular etc.

The delivery of the drug Doxorubcin200,201

into the mononuclear phagocytic

system rich organs/ tissues like liver, spleen and lungs is helpful for treatment of

hepatic-carcinoma, hepatic metastasis, gynecological cancers, bronco pulmonary

tumors, myeleoma and leukemia. Bioactive molecules and vaccines based on peptides

and protein can also be encapsulated and delivered. The insulin loaded nanoparticles

were found to reduce blood glucose for a longer period202

. Polynucleotide vaccines

and therapeutic genes are also delivered to produce immune response203

and treatment

of bone healing204

respectively. Several drugs have also been delivered to the brain by

crossing the blood-brain barrier system, the nanoparticles, containing the drugs, were

transported to the brain by receptor mediated transcytosis205,206

.

i) Nanomedicines

Some of the challenges facing conventional therapies are poor bioavailability

and intrinsic toxicity. These have seriously compromised the therapeutic efficacy of

many otherwise beneficial drugs. Thus innovative nanodevices and nanostructures

have been developed for applications207-209

in the fields of diagnostics, biosensing,

therapeutics, drug delivery and targeting.

Drug delivery with nanotechnological products takes advantage of the patho-

physiological conditions and anatomical changes within diseases tissues to achieve

site specific and targeted delivery. The nanosystems are accumulated at higher

concentration thus enhancing bioavailability at the targeted site. This leads to less

systemic toxicity. The unique property of the nanosystem has helped to deliver drugs

to harder-to target sites such as the brain, where blood-brain barrier exists210

. Some

32

nanoparticles can penetrate smaller capillaries and are taken up by cells. Many of the

NP is known to be biocompatible, undetected by the immune system and

biodegradable. Additional some may possess unique optical and electrical properties

e.g. AuNP, making it possible to track their systemic movement and localization.

Recent advances210

in the use of nanoparticles in medicine include delivery of

antigens for vaccinations, gene delivery for treatment or prevention of genetic

disorders, in cardiac treatment, in dental care and orthopedic applications.

Chrysotherapy the use of gold in medicine has been practiced since antiquity.

Ancient cultures in Egypt, India and China have used gold to treat diseases like

smallpox, skin disorders, syphilis, and measles. In India, Ayurved an ancient medical

science makes use of various Bhasma. One of them Suvarna Bhasma (Gold Ash) is a

therapeutic form of gold metal nanoparticles. In the past few decades, several

organogold complexes have emerged with antitumor, antimicrobial, antimalarial and

anti-HIV activities. It is also used in the treatment of rheumatoid arthritis. Gold

nanoparticles of various shapes have been found to effective in this treatment.

Proteins grafted into gold nanoparticles have also been used to kill cancer cells210

.

The antibacterial effects of Ag salts have been noticed since antiquity211

, and

Ag is currently used to control bacterial growth in a variety of applications212,213

,

including dental work, catheters and burn wounds. It is well know that Ag ions and

Ag-based compounds are highly toxic to micro-organisms, showing strong biocide

effects on a number of bacterial species. The hybrids of Ag nanoparticles with

amphiphilic hyper branched molecules exhibit effective antimicrobial activity in

surface coating agents214,215

.

1.7 Scope of the present work

Nanomaterials have emerged as an area of current interest motivated by

potential applications of these materials in electronics, non-linear optics and magnetic.

Growth in the area has been facilitated by wide accessibility to tools necessary to

characterize these materials. Metal nanoparticles exhibit properties differing from the

bulk metal due to the quantum size effects, including novel electronic optical and

chemical behavior. The properties may be tuned via control of size shape inter-

33

particle spacing and dielectric environment, and methods to vary these have been

developed. In particular nanoparticles made from silver or gold with associated strong

plasmon resonance have increased interest. The plasmon properties of these

nanoparticles of these nanoparticles depend upon their size and shape. Silver

nanoparticles have a potential to eliminate bacteria and fungi, for cosmic and beauty

applications and have self cleaning properties. Gold nanoparticles have a wide variety

of potential applications, for instance, an effective drug delivery in cancerous tumor,

biomedicals, in a wide range of cosmetic and beauty applications. The bimetallic

nanoparticles are of wide interest since they lead to many interesting properties. They

are particularly important in the field of catalysis since they often exhibit better

catalytic activity than their nanometallic counterparts. Both Ag and Au have very

similar lattice constants and are completely miscible over the entire composition

range, hence single phase alloys can be achieved.

In the present work interest in the silver and gold monometallic, bimetallic

clusters and colloids is for fundamental and practical reasons. The particles in the

range of 1 to 10 nm have been seen to have unusual electronic properties which may

lead to new technologies based on advanced materials, e.g., quantum dots in the

miniaturaization of electronic devices. Some progress has been made to control the

particle size of the nanostructured metal clusters. However true control of the particle

size, still remains the most attractive goal in this field.

The electrochemical reduction method has been reported to synthesize metal

colloids and nanoparticles in the nanometer regime and that the particle size can be

controlled in a simple manner by the variation in the current density. Apart from this

important aspect the method is simple, easy to operate, cost effective, allows high

purity of the products, allow reproducible results, and is environment friendly since it

does not make use of toxic chemicals. Thus in the present study the method is selected

for the preparation of monometallic and bimetallic nanoclusters of silver and gold.

In the preparation of nanoclusters capping agents are required to stabilize them

and prevent their aggregation. There are various classes of capping agents available

for stabilization the surfactants are one such class. The surfactants used in the present

34

study are a group of tetra alkyl ammonium salt, which include TEAB, TPAB, TBAB

and TOAB. Except for TOAB there are very few reports regarding the use of other

surfactants. This efficiency in the synthesis of small size nanoparticles by this series

of the surfactant will be evaluated. The tetra alkyl ammonium ion forms a monolayer

around the nanoparticles and is weakly bonded with it. These have thus drawn an

attention for applications which require their labile (or temporary) monolayer binding

properties. Moreover these precipitates are soluble in nonpolar solvents.

The electrochemical reduction method for the synthesis of the nanoparticles

this can either in the form of water or non polar organic solvents. The non polar

organic solvent in the form of a mixture of acetonitirile and tetrahydroforan (4:1) is

used. The advantages in using the non polar organic solvent include control over the

size of the nanoparticles, monodispersity and the chemical nature of the nanoparticles

surface (via capping).

In order to understand the effect of the current density on the size of the

nanoparticles synthesized the current density can be varied. The electrochemical

reduction process will be carried out 6 and 10 mA / cm2 current density.

The samples synthesized in the present work will be subjected to

characterization using techniques like UV-visible spectroscopy, FTIR spectroscopy,

XRD analysis, XPS, SEM and TEM. These analytical techniques will help to

understand a number of important aspects of the synthesized nanoclusters. The UV-

visible study will help to understand the surface plasmon behavior of the

nanoparticles while the FTIR study can reveal if proper capping has been achieved.

The XRD analysis will give information regarding the size and the phase of the metal

nanoparticles. XPS analysis will help to give information regarding the chemical

content of the nanoparticles and the nature of interaction between the metal

nanoparticles and the capping agent. The SEM study will help to reveal the surface

nature of the nanoparticles. The TEM analysis will give information of the shape and

size of the nanoparticles.

Thus the present work can be pursued in a systematic and result oriented

manner with definite information being obtained for the mono metallic and bimetallic

35

nanoclusters of silver and gold capped with different capping agents like TEAB,

TPAB, TBAB and TOAB. Results can be obtained for the effect, on the variation of

the current density as well on the variation of the bukiness of the capping agent on the

synthesis of the nanoparticles.

36

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