Maria Kulsoom

8/3/2019 Maria Kulsoom

1/26

Nano Particle- A particle of matter is normally referred to as an NP if its extension in all

three dimensions is less than 100nm. ( It is about one thousandth of the width of human

hair).Nanocrystallite- A Nanocrystallite is generally understood to possess crystalline order in

addition to nanoscale size, although not necessarily the crystal structure characteristic of the

corresponding bulk material. Clusters- Clusters are particles containing a very small

number of atoms such that it is no longer possible to clearly distinguish bulk atoms

from those at the surface. There is no universally accepted definition but a general rule

is a few hundred atoms or smaller.

Nano composites- A composite is a material that is made up of various components.

Nano gold deposited in the pores of porous alumina membranes is a composite

material.

Colloids-Colloids is a dispersed phase of one substance that exist as discrete entitieswithin a continuous phase, usually water. For example, a colloid made from the bottom

up begins with the nucleation of appropriate atoms or molecules in a supersaturated

solution. When enough colloids are formed, condensation occurs to form three

dimensional structures. The physical conformation of the structure is dependent upon

the size of the colloids and the chemical nature of their surface. Monodisperse- NPs

with a relative standard deviation of the size of distribution of less than 5% are said to

be monodisperse. For example, monodisperse iron nanocrystals with a mean diameter

of 4nm should contain between 2400 and 3300 iron atoms.Reduction-is defined as lossof oxygen, gain of hydrogen or gain of electrons, the gain of electrons enables to

calculate an oxidation state.(One part of a redox process which involves the exchange of

electrons the side which gets reduced receives the electrons and thus reduces its

oxidation state.) Oxidation state-the formal charge on the atom due to its acquisition or

donation of electrons to form bonds.

Emulsifier- A surface-active agent that promotes the formation of an emulsion.

Radical- 1.two or more atoms bound together as a single unit and forming part ofa molecule.

2. An atom or group of atoms with at least one unpaired electron; in the body it is

usually an oxygen molecule that has lost an electron and will stabilize itself by stealing

an electron from a nearby molecule


2/26

Surfactant- Surfactants (soaps and detergents) are surface active chemical agents.Surfactants are compounds or polymers having both hydrophilic and

hydrophobic parts (they are amphiphilic) that reduce interfacial free energy,

such as the surface tension at an air-water interface. Surfactants can be anionic,

cationic, or non-ionic depending on the structure of the polar end.

Passivation- to reduce the chemical reactivity of a surface by applying a coating.

Emulsion- a colloid in which both phases are liquids. Sols- Colloids are what make sols.

A sol is a colloidal suspension of solid particles within a liquid.Gel- A gel is a solid that

contains liquid within its pore structure.Sol-gel chemistry- chemistry based on the

hydrolysis and condensation of suitable organo-metallic precursors to produce metal

oxide networks.Ostwald ripening- In Ostwald ripening larger particles that are

energetically favored due to curvature phenomena grow at the expense of smaller, lessstable particles.Template- A template is a material that acts as a gauge, pattern, or mold

that is used to guide the manufacture of another piece. Micelles-Micelles are made of

molecules called amphiphiles, that is single molecules that have both a polar and a non

polar chemical group. Ligand- A substance (an atom or molecule or radical or ion) that

forms a complex around a central atom.Calcine-Heat a substance so that it oxidizes or

reduces.Precursor- A substance from which another substance is formed. Zeolite-

represent a large family of micro porous tectosilicates having pore sizes smaller than 1.4

nm. Zeolite materials are constructed of negatively charged aluminosilicates hostframeworks that are sufficiently porous to accommodate a variety of different

countercations (as charge compensating ions) and in many cases guest molecules that

can be reversibly adsorbed and desorbed. Zeolites exhibit 3D framework structures

with uniform sized pores of molecular dimensions, typically ranging from 0.3 to 1nm in

diameter, and pore volume ranging from 0.1 to 0.35cc /g.

Self-assembly- apparently spontaneous self organization of objects; it arises as asystem strives to find minimal free energy.

Self-assembly- means that components, constituents, molecules, etc. all cometogether spontaneously without the input of energy or design. Some input

energy is usually required, of course, but it is expressed at new levels of subtlety.

The driving energy may be sequestered in the surface of a nanoparticle, in the

from of a molecular recognition couple or within an excited state of a molecule.


3/26

The making of relatively weak intermolecular bonds are controlled by entropic

trade offs and small H, thus giving the appearance of self assembly.

Hydrolysis- Is the process of breaking bonds by the action of H2O. Condensation- is the formation of a bond with the simultaneous release of a

water molecule.

Pyrolysis -Transformation of a substance produced by the action of heat. Annealing- Hardening something by heat treatment. Sintering- Cause (ores or powdery metals) to become a coherent mass by heating

without melting.

Aerogel- An aerogel is an extremely low-density (porosity>90%) and highinternal surface area (>1000m2g-1) micro porous structure derived from a sol-gel

process in which the liquid component is replaced by gas. Super critical

extraction of the liquid and the subsequent baking form compounds called

aerogels. Xerogel-Xerogel generally have porosity that is much lower and hence

higher density. Extraction of the liquid at non supercritical conditions forms

compound called xerogels.

Dendrimers- Since 1985 a new class of polymers called dendrimers has been developed. Dendrimer molecules are highly branched andlower in molecular weight than most linear polymers. Whereas most polymers are

synthesized by chain reactions or by many repetitions of one type of reaction in

one pot, dendrimers are synthesized one step at a time. Because the branching

confers dendrimers with a more compact shape than other polymers, they are

valuable for construction of organic or hybrid inorganic-organic int erface.

Dispersion medium Dispersed phase Name of system Examples

Gas Liquid Aerosol Fog, mist, clouds

Gas Solid Aerosol Smoke


4/26

Liquid Gas Foam Whipped cream

Liquid Liquid Emulsion Milk, mayonnaise

Liquid Solid Sol Gold in water

Solid Liquid Gel Ruby glass, gold in

glass

Solid GAs Solid foam Pumice, Styrofoam

Nano particles (NPs) form a new class of materials possessing unique propertiesthat are characteristics of neither the molecular nor the bulk solid state limits.

They have become the focus of considerable fundamental and applied research

leading to important technological applications in areas such as heterogeneous

catalysis, optical communications, gas sensing, nano electronics, and medicine.

NPs come in wide range of sizes and shapes, with varied electronic, optical and

chemical properties. However, the properties of NPs are intimately connected to

their nanoscale size and atomic scale structure.Almost every element in the

periodic table, together with various alloys and compound can form NPs.NPs are

also ubiquitous in nature, for example, as soot particles in the atmosphere and

soot in interstellar space, and they are even produced by certain types of

bacteria.One often distinguishes between two types of Nano particle structures:

those of low potential energy, which are close to thermodynamic equilibrium,

and those of higher potential energy, which are formed by kinetically limited

processes. It is often possible to transform the latter into the former by suitable

thermal annealing; however, kinetically controlled structures can be preferable

for application if they exhibit structure features with desirable properties.


5/26

The distinguishing feature of NPs, irrespective of their detailed structure, is that they

possess a large surface area relative to their volume and a large fraction of atoms that

are under-coordinated compared to the bulk. These surface atoms are responsible for

many of the unique properties of NPs and some insights into their structure can be

gained by considering macroscopic surfaces.

The techniques for the creation of nanostructures can be divided into two broadcategories. Top down approaches use lithographic patterning to structure

macroscopic materials at the nanoscale, such as electron beam lithography,

atomic beam holography, scanning probe lithography. Bottom up approaches

utilize growth and self assembly to build nanostructures from atomic or

molecular precursors. Some non-lithography techniques are natural self

organized epitaxial growth, chemical synthesis of colloidal nanostructures,

synthesis of nanostructures in glass and polymer materials, and template based

chemical and electro chemical synthesis of nano structures. It is typically difficult

to create structures smaller than 50nm with top-down technique, while it is often

difficult to create structures larger than 50 nm by bottom up technique.No single

fabrication/synthesis method is universally applicable for the production of nano

meter scale semiconductor devices.Probably the most useful methods of

synthesis in terms of their potential to be scaled up are chemical methods.

Bottom up fabrication approaches selectively combine atoms or molecules toform nanomaterials. Bottom up fabrication methods, therefore , are considered to

be additive. Bottom up fabrication methods reside within the realm of chemistry

and biology. Nature, of course, has perfected bottom up fabrication of

nanomaterials.

Advantages of bottom up methods are numerous. Self-assembly processes, forexample, occur under thermodynamic control conditions. Because such

processes exploit much weaker intermolecular interactions, as opposed to strong

covalent bonds, nanomaterials are fabricated under milder conditions of

temperature, pressure, and pH. The upscale potential of bottom up methods is

enormous. As with any other chemical process, it is relatively straightforward to

scale up a process that takes place in a beaker of on a lab bench (e.g., the domain

of the chemist) to a batch production process in a manufacturing line (e.g., the

domain of the chemical engineer). However, there exist significant challenges


6/26

facing bottom up methods. Overall robustness, long range order (related to

complicated patterns), and directed growth all leave something to be desired. In

order for bottom up fabrication of nanomaterials to become the dominant

fabrication mode of industry, all of these concerns need to be overcome. We

divide bottom up methods according to the phase within which the process

occurs e.g., Gaseous-Phase method, Liquid-phase methods, and solid phase

bottom up fabrication.

METAL NANOCRYSTALS BY REDUCTION

Reduction of metals, particularly gold salts like hydrogenated tetrachloroaurateby organic bases such as sodium citrate is a very old procedure. As the solution

becomes saturated, Au nanoparticles nucleate and start to precipitate. Control of

solution parameters such aspH, concentration of reducing agent, and potential

stabilizing ligands lead to control over particle size. Reaction scheme of

formation of Au55 ligand-stabilized cluster. At the top left, a solution containing

dissolved metal cations is shown. The cations are converted into gold atoms after

addition of reducing agent like citrate. Once formed, the atoms nucleate and

grow into aggregates that eventually stop at the cluster phase (depending upon

the reaction conditions). Ligands attach to the Au55 vertices of the cluster; there

are 12 vertices in this structure. Not shown are the counter anions, the chloride

atoms of Au55[P(Ph)3]12Cl6.


7/26

Au clusters

One of the most popular methods to form metal clusters and colloids is throughthe reduction of metal cations. Common reducing agents include the base

complements of organic acids such as sodium citrate, reducing alcohols, Na2S,

borohydrides [B2H6], sodium borohydride [NaBH4], and even hydrogen gas.

The small clusters wish to agglomerate to form larger clusters via Ostwald

ripening. In order to fabricate metal nano clusters of predetermined size, special

steps need to be taken. First , addition of a potential ligand species to the

reaction mixture is required. The ligand serves to bind reduced metals and

thereby modulate the growth of the embryonic clusters. Depending on relative

concentrations of reactants (the metal salt, reducing agent and ligand), growth of

clusters that are mono disperse with desired dimensions is possible. The generic

process for synthesis of nano clusters is summarized as

Reduction of metal cation agglomeration prevention ligand stabilization orligand exchange extraction from solvent further surface modification.

Gold colloid synthesis is straight forward. Of all the procedures available today,a method called the Turkevitch route is very popular


8/26

HAuCl4+(C6H5O7)Na3 Au + Oxidized products

Approximately 5X10-6 mol of HAuCl4 is dissolved in 19mL of deionized water and

heated to boiling; 1mL of 0.5% sodium citrate is added with constant stirring for 30 min.

The solution undergoes color changes from yellow to clear gray, purple, deep purple,and finally to ruby-red. Water is added to maintain the level of solution to 20 mL.The

Brust route is similar but employs sodium borohydride as the reducing agent:

HAuCl4+[CH3(CH2)7]4NBr(TOAB)+toluene+NaBH4Au

This technique utilizes an emulsion layer made of water and toluene; 4.0X10-3 mol of

tetraoctylammonium bromide (TOAB, surfactant, the phase transfer catalyst and the

stabilizing ligand) is added to 80mL of water and then 9.0X10-4 mol of HAuCl4 in 30

mL water is added to the TOAB solution and stirred vigorously of 10 min. The aqueous

phase is clear and the organic phase is orange. Sodium borohydride is added drop wise

to the mixture and the color changes for orange to white to purple to dark red. To make

sure that the product clusters are monodisperse with regard to size, the solution is

stirred for an additional 24hr. The organic phase is washed with sulfuric acid to

neutralize the solution. TOAB is not considered to be a strong ligand and will readily

undergo ligand exchange with stronger ligands like thiols that bind covalently to the

gold clusters.Nano particles of Molybdenum (Mo) can be reduced in toluene solution

with NaBEt3H at room temperature, providing a high yield of Mo nano particles having

dimensions of 1-5 nm. The equation for the reaction is

MoCl3+3NaBEt3HMo+3NaCl+3BET3+(3/2)H2

where Et denotes the ethyl radical (C2H5).* Nano particles of aluminum have been

made by decomposing Me2EtNAIH3 in toluene and heating the solution to 105C for

2hr (Me is methyl,CH3). Titanium isopropoxide is added to the solution. The titanium

acts as a catalyst for the reaction. The choice of catalyst determines the size of the

particles produced. For instance 80 nm particles have been made using titanium. A

surfactant such as oleic acid can be added to the solution to coat particles and prevent

aggregation.The solvothermal and hydrothermal syntheses involve heating the

reactants in water or a solvent at high temperatures and pressures. The role of solvent

or water is that of pressure-transmitting medium and the solubility of the reactants is

pressure and temperature dependent. A sealable Teflon-lined container, called a bomb,

is used to keep the solvent and the reactants inside. After sealing, the container is kept


9/26

at high temperatures inside an oven (temperatures vary from 100 C to 500 C). The

pressure of the container depends on the level of filling of the solvent or water.

Solvothermal (hydrothermal) conditions provide unique supercritical conditions that

can lead to unique or unexpected morphologies of the products. The method is simple,

economical, robust, and most of the time the conversion efficiency is close to 100%.

Various experimental parameters such as concentration of reagents, pH, and

introduction to additives can be varied to tune the morphologies of the products. The

effect of various experimental parameters on the reaction equilibria seems to be the key.

Solvothermal and hydrothermal syntheses have been used to synthesize a variety of

nanorods which are summarized in Table 7.2.

Solvothermal synthesis

During solvothermal synthesis, sometimes layered precursors are used as the startingmaterials and template molecules such as amines are used. The layered precursors with

template amine molecules in the interlamellar space on solvothermal treatment lead to

the transformation of a two dimensional structure into a one-dimensional structure.

Simple conditions such as using different acidic solvents (e.g. H2SO4, HCl, salicylic

acid) can lead to nanorods of different aspect ratio. The major drawback of these

methods is that the mechanism of synthesis is sometimes not clearly established and

reproducibility may be an issue.Nickel nanorods (diameter 12 to 15 nm; length, 50 to

100nm) have been synthesized by a solvothermal decomposition of nickel acetate in thepresence of n-octylamine (nickel acetate to n-octylamine molar ratio is 1:300) at 250C

(104). The formation of Ni nanorods id favored by the presence of n-octyl amine; it

reduces , under solvothermal conditions, the Ni2+ ions to Ni and also acts as a shape-

controlling agent to produce metallic nickel nanorods. In the absence of linear alkyl

amines, only NiO nanoparticles are produced. Using a similar approach, in the presence

of n-octylamine, nanorods of ruthenium and rhodium metals have been produced

stating from corresponding acetyl acetonate precursors, Ru(acac)3 . The metallic

nanorods are stable in air because of amine coating and can be redispersed in

hydrocarbon solvents. Photochemical synthesis may be considered as one pot

synthesis. It is mild, efficient, and an environmentally friendly method. Gold nanorods

with control over aspect ratio can be synthesized using photochemistry in the presence

of Ag+ ions. An aqueous solution containing CTAB [Cetyl trimethyl ammonium

bromide] and tetradodecyl ammonium bromide has been used as the growth solution


10/26

along with HAuCl4.3H2O as the source of gold. A small amount of acetone and

cyclohexane is used to loosen the micellar structure. Different amounts of 0.01M

AgNO3 aqueous solution is added to the above solution and it is irradiated with UV

light (~254nm). The resulting precipitate after dispersion in water yields Au nanorods.

The formation of gold nanorods is indicated by the appearance of a longitudinal band

in the UV-Vis spectrum around 600 to 800nm. Depending on the amount of Ag+ ions

(15.8 to32l), Au nanorods with aspect ratio 2.8 t0 4.8 can be synthesized. Silver ions

plays a crucial role in the formation of nanorods and a sample prepared without silver

nitrate consists of only spherical particles Increase in Ag+ content in the solution leads

to a decrease in the diameter of the rods. A combination of crystal aggregation and

stabilization of a particular crystal face has been suggested to be the mechanism. Silver

ions are reduced to silver Nanocrystals when irradiated and they are oxidized back into

Ag+ in the presence of HAuCl- 4:Au3+ gets reduce in the process. This leads to freshsurfaces of Au nanocrystals followed by growth along a particular direction.

A photochemical reaction of ketone to synthesize gold nanorods in a micellarsolution of CTAB consisting pf HAuCl4, AgNO3, acetone, And ascorbic acid is

possible. The presence of ketone is crucial for the formation of the rods in this

case. The UV irradiation leads to the formation of ketyl radials and the radicals

reduce the Au+ ions to Au. The Au act as the nuclei for the anisotropic growth

of Au nanocrystals in the presence of ascorbic acid and the growth solution ,

consisting of AuBr2-, AgBr clusters, and CTAB. The proposed mechanism is

given below;

CTAB is denoted by RH and R*is the radical generated from CTAB by hydrogen

abstraction by (CH3)2*COH which reduces Au+ to Au. The energy and intensity of the

UV light used during the growth of gold nanorods has been studied. Under similar

experimental conditions (aqueous solution consisting of CTAB, tetraoctyl ammonium

bromide, H AuCl4.3H2O, AgNO3, acetone, cyclohexane is used) 300 nm UV light

)(

)()(

)()(

)()(

0

23

0*

23

**

23

*

23

*

2323

3

NanorodsAunAu

HCOCHAuCOHCHAu

RCOHCHRHCOCH

COCHCOCH

AuAu

n

h

AcidAscorbic


11/26

produces longer nanorods with narrower size distribution as compared to irradiation

with 254nm light. Also, high intensity accelerates the growth of Au nanorods. Longer

irradiation time reduces the concentration of rods, with a concordant increase in the

number of spherical particles. The advantage of the photochemical method compared to

the electrochemical method is that spherical particles are not present.

Electrochemical synthesis

Electrochemical methods have been mainly used to deposit metals or semiconductors into the templates. One does not need expensiveinstrumentation, and the synthesis can be carried out under ordinary temperature and pressures. A general scheme for the synthesis of

nanorods by electrochemical method is given in scheme below. First a thin film of metal is deposited on the template, which w ill serve as

the working electrode for the deposition. This is followed by the deposition of the sacrificial metal. Then the deposition of the materials of

interest is carried out electrochemically. Finally, the templates are removed by chemical treatment to get nanorods.

Martin and coworkers pioneered the electrochemical deposition of nanorods of metals such as Ag, Au, Co, Cu, Ni, Pt, Pd, and Zn usinghard templates such as anodic aluminium oxide (AAO). The metal ions in the solutions are reduced by applying a negative poten tial and

the morphology of the rods is controlled by two parameters, the pore size of the template, and the amount of the charge passed, since the

length of the nanorods will be decided by this. It is possible to deposit multi elements (e.g. grow multi segmented rods) within pores of the

template. One can adopt a pulsed electrochemical deposition with a bath containing multiple ions with well-separated redox potentials. It

is also possible to deposit semiconductors into the pores of the template; for example ZnO nanorods or nano wires can be prepared by

applying a cathodic current to an aqueous solution con taining zinc nitrate.

The major drawback of the electrochemical methods are (1) modification of the template electrochemically, for example, plating to make ita working electrode is inconvenient, and (2) to make nanorods, the metal ions in the solution should be easily reducible.; if we cannot

reduce a metal electrode chemically, this approach cannot be used.


12/26

Thermolysis

Nano particle can be made by decomposing solids at high temperature havingmetal cations, and molecular anions or metal organic compounds. The process is

called thermolysis. For example, small lithium particles can be made bydecomposing lithium azide, LiN3. The material is placed in an evacuated quartz

tube and heated to 400C in the apparatus as shown in Fig. At about 370C the

LiN3 decomposes, releasing N2 gas, which is observed by an increase in the

pressure on the vacuum gauge. In a few minutes the pressure drops back to its

original low value, indicating small colloidal metal particles. Particles is less than

5nm can be made by this method. Passivation can be achieved by introducing an

appropriate gas.

The presence of these nanoparticles can be detected by electron paramagneticresonance (EPR) of the conduction electrons of the metal particles. EPR measures

the energy absorbed when em radiation such as microwaves induces a transition

between the spin states ms split by a DC magnetic field. Generally the

experiment measures the derivative of the absorption as a function of an

increasing DC magnetic field. Normally because of the low penetration depth of


13/26

the microwaves into a metal , it is not possible observe the EPR of the conduction

electrons.

However, in a collection of nano particles there is a large increase in surface area, and

the size is of the order of penetration depth, so it is possible to detect the EPR of theconduction electrons. Generally EPR derivative signals are quite symmetric, but for the

case of conduction electrons, relaxation effects make the lines very asymmetric, and the

extent of asymmetry is related to the small dimensions of the particles

Hybrid method

A nano scale hybrid inorganicorganic material is defined as a material havingproperties that depend on the size of at least one component. The organic part of

most hybrid materials is not crystalline but amorphous, usually a surfactant or

polymer. Often a surfactant or polymer coating is necessary to prevent nanoscale

inorganic materials from coagulating into bulk materials. Polymer composites, in

which dispersed inorganic particles or fibers improve the mechanical properties

of a plastic or an elastomer, have been commercial since the first carbon black


14/26

filled rubber tires.Inorganic-organic materials are ubiquitous on the nano-scale,

sometimes by design in order to combine the separate properties of the inorganic

and organic components into one material, and sometimes by necessity because

an organic surface is the only way an inorganic material can be dispersed in a

solvent for processing.Organic materials serve as templates for the synthesis of

nanoparticles, such as TOPO- stabilized II-IV semiconductor nanoparticles, for

the ordering of inorganic nanoparticles in dendrimers and block copolymers, and

for the synthesis of nano-porous zeolites. The organic templates often is removed

to obtain a final inorganic product such as a zeolite, or an ordered array of

nanoparticles on a surface, and an inverse opal photonic band gap material.

Because the inorganic-organic interface must be stabilized to maintain nano

structures, surfactants and polymers will continue to be essential components of

many new hybrid materials.

Semiconductor nanoparticles

The most nearly monodisperse, stable, and highest quantum efficiency ofnanoparticles of II-Vi semiconductors, such as CdSe, are produced in non-polar

organic media at temperatures of 250C to 300C with surface coatings of

trioctylphosphine oxide [TOPO]. Often a protective inert shell of a large band

gap material (silicon dioxide, zinc oxide, or zinc sulfide) is grown on the surface

to retard reactions of the smaller band gap nanoparticle with the environment.Aqueous dispersions are produced either by replacing the TOPO with a

hydrophilic stabilizer, or by carrying out the synthesis under aqueous conditions

in the first place using stabilizers such as 2-mercaptoglycolic acid (HSCH2CO2H)

or 2-mercaptoethanol (HSCH2CH2OH). Colloidal stability in water often

requires a large excess of the water-soluble stabilizer in solution, which

equilibrates with a much smaller amount of stabilizer on the surface of the

particles. In contrast to low molecular weight stabilizers, a large excess of the

water-soluble stabilizer is not required because the many bonding sites on the

polymer add up to much stronger binding of a polymer than of a monodentate

ligand. Aqueous dispersions of nanoparticles stabilized initially by 2-

mercaptoglycolic acid and 2-mercaptoethanol have poorer optical properties

than the TOPO-stabilized nanoparticles due to broader particle size distribution,

which result in broader emission bands and surface trapped states that reduce


15/26

the quantum yield of fluorescence. In toluene-water mixtures, 4.6 nm diameter

TOPO-coated CdSe particles form a disordered monolayer at a water toluene

interface, but not at a toluene-air interface.

Template synthesis is the fabrication of nanomaterials within porous materialsand interstitial spaces. We focus primarily on porous materials formed by

inorganic and organic inorganic hybrid materials. According to the IUPAC

definition, there exists three classifications of porous materials: macro porous

(d>50nm), mesoporous (2


16/26

A generalized cooperative mechanism of formation was proposed based on thespecific electrostatic interactions between an inorganic precursor Iand a

surfactant head group S. The hybrid inorganic-surfactant mesophases obtained

are strongly dependent on the interactions between the surfactants and the

inorganic precursors (i.e., the hybrid interface).

In the case of ionic surfactants, the formation of the mesostructured material ismainly governed by electrostatic interactions. In the simplest case, the charges of

the surfactant (S) and the inorganic species (I) are opposite under the synthesis

conditions. Along with the S+I- interaction, cooperative interaction between

inorganic and organic species can also be achieved by using the reverse charge

matching, interactions S-I+. With these two direct synthesis routes identified, and

, two other synthesis paths, considered to be indirect, also yield hybrid

mesophases from the self assembly of inorganic and surfactant species. Synthesis

routes involving interactions between surfactants and inorganic ions with similar

charges are possible through the mediation of ions with the opposite charge (S+

X-I+or S-M+I-). The S+ X-I+ path takes place under acidic conditions, in the presence

of halide anions (X- = Cl-, Br-) and the S-M+I- route is characteristic of basic

media, in the presence of alkali metal ions (M+ =Na+, K+). Besides the syntheses

based on ionic interactions, the assembly approach has been extended to

pathways using neutral (S 0) (87) or non ionic surfactants (N0) (88). In the

approaches denoted by (S0I0) and (N0I0), hydrogen is to be considered to be main

driving force for the formation of the mesophase.

Sol gel method

Sol-gel synthesis is one of the oldest forms of nanotechnology that has incrediblepotential for nano manufacturing. Colloid chemist have developed this

technology dating back numerous decades, developing one of the simplest,

inexpensive, low temperature, and most effective bottom up wet chemical

synthesis of nano materials that are highly pure and monodisperse in size.

Inorganic metal oxides as well as inorganic-organic hybrid materials are

synthesized routinely by sol-gel methods. The sol is the homogeneous solution

of molecular reactant precursor that are concerted into an infinite molecular

weight three-dimensional polymer: the gel that forms an elastic-solid fill material

with the same volume as the liquid. Mixtures of precursors (different metals,


17/26

oxides, and even organics) lead to binary, ternary, and higher order systems.

Colloid size, structure and morphology are dependent on the solution pH,

temperature, composition, concentrations and the solvent. Transformation from

the sol into the gel, for example, is stimulated by a change in the pH of the

solution. Sol-gel synthesis proceeds by hydrolysis and condensation of silicate

precursors like triethlorthosilicate (TEOS). Hydrolysis of a silicon precursor

proceed where R is an ethyl group in TEOS. Subsequent condensation is

illustrated by:

Using sol-gel synthesis zero , one, two dimensional and higher order

structures and morphologies can be synthesized.. Different metal groups and

combinations of metals like aluminium, potassium, titanium, and others result in

colloidal ( and eventually ceramic ) materials of great diversity, properties and

function.Metal nanoparticles are often synthesized on inorganic supports. Sol-gel

processes in this way; contribute to the synthesis of metal zero-dimensional

materials. The inorganic support serves to isolate metal precursors before

reduction and thereby prevent agglomeration of the metal. The sol-gel product

also serves in the capacity of template.

Starting from the absolute bottom with molecules and via the process ofnucleation and Ostwald ripening, larger and larger particles are grown until the

reaction is terminated. Following a sintering process, an array of close-packed

spherical particles can be used to form aero gels or xerogels or act as template to

form other nanoparticles.

Through sol-gel method mono disperse particles can be generated in largequantities. Most sol-gel systems are based on silicate chemistry but others exist as

well, such as the formation of mono disperse hematite from ferric

hydroxides/ferric oxy hydroxides. Sol-gel chemistry is exploited to form silicananoparticles as drug delivery vehicles.

Sol-Gel method is a good method for obtaining fine nanoparticle with narrowsize distribution and controlled chemical composition at relatively low

temperature. In this process, dispersion of the particles of the metal compound

(sol) usually in an aqueous phase is first prepared and then converted into gel.

yROHOHORSiOyHORSi yy )()()( 424

OHOHORSiOSiOHROOHORSi yyyyyy 214144 ])()[(_])()[()()(2


18/26

The polymer gel so formed is three dimensional skeleton surrounding the inter

connected pores. Gelation is achieved by means of chemical dehydrating agent

with surfactant. 2-ethylhexanol (dehyrating agent) and SPM-80 (surfactant) are

commonly used in the process. Typically this involves a hydrolysis reaction

followed by condensation or polymerization. In principle, particle size achieved

at the precipitation state is maintained during sol and gel formation.

Si(OC2H5)4+C2H5OH(SiO2)n (gel)+Other products

The sol gel process is summarized in fig. below. Steps 1 to 3 indicate the processup to gelation. While the gelled spheres or collapsed gels (xerogels) can be

collected, a better means of exploiting their surface area is to capture the gel on

the surface. This way a greater surface to bulk area is obtained. Another

possibility is aero gels. Aerogels are composed of three dimensional, continuous

networks of particles with air (or any other gas) trapped at their intersection.

Aerogels are porous and extremely light, yet they can withstand 100 times their

weight. Another very clever way of maximizing the surface area is colloidal

crystallization on surfaces. In this process, water is very carefully removed so

that the sol gel structure is not lost in the precipitate. Nanostructured silica with

controlled pore size, shape and ordering can be obtained this way. When

surfactants are mixed with water, long range spatially periodic architectures are

created in a nano foam, with lattice parameters in the range of 2nm to 15nm.

To obtain nanoparticles in a bulk form the nanoparticles must be consolidatedwhile maintaining the nano size. The usual goal is to form a high density solid

that is free of voids. Sintering is a common technique for consolidating metallic

and ceramic materials. The material is first compacted into a low density solid

which may contain binders. Next, higher temperatures and sometimes pressures

are used to increase the density by the diffusion of vacancies out of the pores.

Compacted nano crystalline metals can have densities which are 96-98% of the

bulk. In sol-gel synthesis, a sol is prepared first from the suitable precursors. The

sol becomes a gel on aging. The gel after drying and calcination leads to

nanorods. The sol-gel process has been used mainly for the synthesis of metal

oxide nanorods, with a few exceptions. The advantage of sol-gel process

compared to other methods are (1) better composition control, (2) good

homogeneity, (3) low processing temperature, (4) and adaptability for easier


19/26

fabrication. Typical examples of nanorods synthesized by the sol-gel process

along with the chemicals used are summarized in table below. We illustrate two

examples of sol-gel synthesis; Nanorods of K2Ti4O9 and K2 Ti6O13 have been

synthesized by mixing CH3OK and Ti(OC2H5)$ in ethanol. The molar ratio of

CH3OK to Ti(OC2H5)4 is varied. A required volume of HCL is added to control

the hydrolysis and condensation reactions. The sol after keeping for about 4 to 5

days is dried to get a xerogel. The xerogel on calcination leads to K2Ti4O9

nanorods if the ratio of CH3OK to Ti(OC2H5)4 is 1:1. If the CH3OK to

Ti(OC2H5)4 ratio is 1:2, the final product is K2Ti6O13 nanorods. The second

example is the synthesis of-alumina nanorods from boehmite nanofibers using

a modified sol-gel process. First, a solution of aluminium isopropoxide in

anhydrous ethanol is prepared. To this, ethanol with 4% water is added leading

to a viscous liquid after 15h. The viscous liquid heated at 600 C leads to -alumina nanorods (diameter


20/26

Miscelles and micro emulsions

Surfactants (soaps and detergents) are surface active chemical agents. Surfactantsare compounds or polymers having both hydrophilic and hydrophobic parts

(they are amphiphilic) that reduce interfacial free energy, such as the surface

tension at an air-water interface. Surfactants can be anionic, cationic, or non-ionic

depending on the structure of the polar end. The hydrophobic end dissolves oil

or grease, and the hydrophilic end dissolves in water. Surfactants dissolve

molecularly in water at low concentrations. As the concentration increases

critical micelle concentration (CMC) is reached at which further added surfactant

forms aggregates called micelles. The simplest amphiphilic structure is the

micelle. Although micelles are homogeneous in the sense that they consist of the

same molecule, but are considered as supra-molecular structures because the

molecular components of micelles are held together by the intermolecular forces.

Assuming that all amphiphiles in a solution are identical, predictions about their

capcapLA

Vcpp


21/26

structure are made by application of the critical packing parameter. The cpp is

given by

Where V is the volume of the amphiphile, Acap is the area of the head group,and L

cap

is the length of the head group. Packing shapes vary from cone like tocylinder like to wedge like. For single tail surfactants that have relatively large

head groups (e.g., sodium dodecyl sulfate, SDS), cpp is equal to 1/3 and the

packing shape is conical with the tails petering out into a point. Spherical

micelles conform to these criteria as well. Single tailed amphiphiles with small

head groups (e.g., hexadecltrimethylammonium bromide, CTAB) pack into a

truncated cone with cpp between 1/3 and and show incipient cylindrical

tendencies. Double tailed amphiphiles with large heads (e.g., phosphatidic acid)

form truncated cones (cpp=1/2-1) that in turn form flexible bilayers. Double tailed

amphiphiles with small heads (e.g., phosphatidyl serine) form cylinders (cpp=1)

that settle eventually into planer bilayers (like membranes) or into inverted

truncated cones (wedges). In the cases where cpp>1, inverted micelles (reverse

micelles) are formed under the proper solution conditions (e.g., phosphatidyl

ethanol amine). An emulsion is a cloudy colloidal system of micron size droplets

of one immiscible (i.e., non mixing) liquid dispersed in another, such as oil in

water. It is formed by rigorous stirring, and is thermodynamically unstable

because the sizes of the droplets tend to grow with time. If surfactants are

present, then nano sized particles, ~100nm, can spontaneously form as a

thermodynamically stable, transparent micro emulsion that persists for a long

time. Surfactant molecules can organize in various ways, depending on their

concentration. For low concentrations they can adsorb at an air-water interface.

Bacteria and nanoparticles synthesis

Among the microorganisms, prokaryotic bacteria have received the mostattention in the area of biosynthesis of nanoparticles. Early studies reveal that

Bacillus subtilis 168 is able to reduce Au3+ ions to produce octahedral gold

particles of nanoscale dimensions (5-25nm) within bacterial cells by incubation of

the cells with gold chloride, under ambient temperature and pressure conditions.

Organic phosphate compounds play a role in the in vitro development of

octahedral Au, possibly as bacteria-Au-complexing agents. Fe(III)- reducing

bacteria Shewanella algae can reduce Au(III) ions in anaerobic environments. In


22/26

the presence of S. algae and hydrogen gas, the Au ions are completely reduced,

which results in the formation of 10 to 20 nm gold particles. It is already

established that silver is highly toxic to most microbial cells. Nonetheless several

bacterial strains are reported as silver resistant and may even accumulate silver

at the cell wall to as much as 25% of the dry weight biomass, thus suggesting the

use for the industrial recovery of silver form ore material. The silver resistant

bacterial strain Pseudomonas stutzeri AG259 accumulates silver nanoparticles,

along with some silver sulfide, in the cell where particle size ranges from 35 to

46nm. Larger particles are formed when P. stuteri AG259, isolated from a silver

mine, is placed in a concentrated aqueous solution of silver nitrate.

Nanoparticles of well defined size, ranging from a few to 200nm or more, and

distinct morphology are deposited within the periplasmic space of the bacteria.

Cell growth and metal incubation conditions may be the reasons for theformation of different particle sizes. The exact reaction mechanisms leading to

the formation of silver nanoparticles by this species of silver resistant bacteria is

yet to be elucidated. The ability of microorganisms to grow in the presence of

high metal concentrations might result from specific mechanisms of resistance.

Such mechanism include the following; efflux systems, alteration of solubility

and toxicity by changes in the redox state of the metal ions, extracellular

complexation or precipitation of metals, and the lack of specific metal transport

systems. Bacteria not normally exposed to large concentrations of metal ions mayalso be used to grow nanoparticles. The exposure of Lactobacillus strains, which

are present in buttermilk, to silver and gold ions resulted to the large-scale

production of metal nanoparticles within the bacteria cells. Moreover, the

exposure of lactic acid bacteria present in the whey of buttermilk to mixtures of

gold and silver ions can be used to grow alloy nanoparticles of gold and silver.In

addition to gold and silver nanoparticles, there is much attention in the

development of protocol for the synthesis of semiconductors (the so called

quantum dots) such as CdS, ZnS, and PbS. These luminescent quantum dots are

emerging as a new class of materials for biological detection and cell imaging,

based on the conjugation of semiconducting quantum dots and biorecognition

molecules. Clostridium thermoaceticum precipitates CdS at the cell surface as

well as in the medium from CdCl2 in the presence of cysteine hydrochloride in

the growth medium. Most probably, cysteine acts as the source of sulfide. When


23/26

Klebsiella aerogenes is exposed to Cd2+ ions in the growth medium, 20 t0 200nm

CdS formed on the cell surface. The formation of CdS is confirmed by

quantitative energy dispersive x-ray analysis. The buffer composition of the

growth mechanism plays an important role in forming the cadmium sulfide

crystallite. Intracellular CdS nanocrystals, composed of wurtzite crystal phase of

the cells and increases about 20 fold in E coli grown in the stationary phase as

compared with that grown in the late logarithmic phase. Spherical aggregates of

2 to 5 nm diameter sphalerite (ZnS) particles are formed within natural biofilms

dominated by sulfate reducing bacteria of the family Desulfobacteriaceae. A

combination of geochemical and microbial processes leads to ZnS bio-

mineralization in a complex natural system. It is appropriate to mention that the

concentration of Zn can be significantly reduced to below acceptable levels for

drinking water with the use of this method. Magnetotactic bacteria are aheterogeneous group of prokaryotes. They orient and migrate along geomagnetic

field lines. All magnetotactic bacteria contain magnetosomes, which are

intracellular structures comprising magnetic iron mineral crystals enveloped by a

membrane vesicle. Magnetic Fe sulfide nanoparticles are synthesized by using

sulfate-reducing bacteria where particles having a size of a few nanometers are

formed on the surface, and the magnetic mineral is separated from the solution

by a high-gradient magnetic field of 1T. Bacterially produced iron sulfide is an

adsorbent for a wide range of heavy metals and some anions. Magnetite is acommon product of bacterial iron reduction and could be a potential physical

indicator of biological activity in geological settings. Single domain tiny magnetic

particles (


24/26

(Fe3O4) that are subsequently assembled into folded chain and flux closure ring

morphologies. The magnetic crystals with large magnetic moments, when

constrained to lie on a two dimensional surface, are responsible for the head-to-

tail assembly. Based on the magnetization measurements and the magnitude of

the magnetization field, it is established that biogenic magnetite nanoparticles

are not superparamagnetic. Magnetic nanoparticles are also assembled into

ordered structures when the motion of the magnetic bacteria M.

magnetotacticum (MS-1) is controlled by applying a magnetic field. After

assembling the bacteria with microelectromagnets, the cellular membranes of the

bacteria are removed by cell lysis to leave the biogenic magnetic nanoparticles at

desired locations. Different patterns of magnetic structures are observed after

removing the cellular membrane of tapped MS-1 bacteria. These types of ordered

magnetic structures could serve as a system for studying the interactionsbetween closely spaced magnetic nanoparticles. Thus, a different approach, the

combination of biomineralization and micromanipulation, can be anew

procedure for growing and assembling nanoparticles into customised structures.

All magnetotactic bacteria contain magnetosomes, which are intracellular

structures comprising magnetic iron mineral crystals enveloped by a membrane

vesicle. The magnetosome membrane (MM) is most likely a structural entity that

anchors the crystal at particular locations in the cell, as well as the locus of

biological control over the nucleation and growth of the magnetosome crystals.Knowledge of biochemical and genetic control on magnetic production is

essential to understanding how the magnetotactic bacteria produce

magnetosomes and organize them in chains. The biomineralization of

magnetosome particles is achieved by a complex mechanism that involves the

uptake and accumulation of iron and the deposition of the mineral particle with

a specific size and morphology within a specific section provided by the MM.

Since the MM is thought to be of paramount importance in magnetosome

formation, researchers have focused on the role of MM proteins, which occur in

the MM but not in the soluble (periplasmic or cytoplasmic) fraction or in the

cytoplasmic or outer membranes, in magnetosome synthesis. The mam (MM)

genes appear to be conserved in a large gene cluster within several magnetotactic

bacteria (Magnetospirillum species and strain MC-1) and may be involved in

magnetic biomineralization.


25/26

The mono dispersity of the silver/gold nanoparticles produced either intra- orextracellularly by the above mentioned methodology in not very high and far

inferior to that obtained through conventional chemical methods. More by

chance than by design, it is observed that alkalothermophilic (extremophilic)

actinomycete, Thermomonospora sp., when exposed to gold ions, reduce metal

ions extracellularly, yielding gold nanoparticles with much polydispersity. A

complete reduction of the 10-3 M aqueous HAuCl4 solution at pH 9.0 and 50C

results to spherical and reasonably monodisperse nanoparticles. In contrast,

intracellular synthesis of gold nanoparticles occurs in alkalotolerant

actinomycete Rhodococcus sp, where particles are more concentrated on the

cytoplasmic membrane that on the cell wall.

Y east and nanoparticles synthesis

Among the eukaryotes, yeasts are explored mostly in the biosynthesis of thesemiconductor nanoparticles. Exposure of Candida glabrata to Cd2+ ions leads to

the intracellular formation of CdS quantum dots. The synthesis of Phytochelation

(PC) is activated in the presence of Cd. The strucutre of PC involves a repeating

sequence of-glutamyl-cysteine pairs to give polypeptides the general formula

(Glu-Cys)n Gly, with n values commonly ranging from 2 to 6. They bind Cd

ions immediately, forming Cd-Pc complexes that are transported into the

vacuole. Then, the complex is degraded and the nanoparticles are formed.Torulopsis sp., which was found in an extensive screening program, is capable of

synthesizing PbS nanocrystals intracellularly when challenged with Pb2+.

Crystallites, which are extracted from the biomass by freeze thawing, exhibit a

sharp absorption maximum ~330nm and are 2-5nm in size.

CdS quantum dots synthesized intracellularly in Schizosaccharomyces pombeyeast cells exhibit ideal diode characteristics. Biogenic CdS nanoparticles in the

size range 1-1.5 nm have been used in the fabrication of a heterojunction with

poly(p-phenylenevinylene). Such a diode exhibits an approximately 75-mA/cm2

current in the forward bias mode at 10V, while breakdown occurred at 16V in the

reverse direction.

Fungi and nanoparticle synthesis


26/26

The use of fungi in nanoparticle synthesis is potentially exciting since theysecrete large amounts of enzymes and are simpler to deal with in the laboratory.

However, the genetic manipulation of eukaryotic organisms as a means of over-

expressing specific enzymes identified in nanomaterial synthesis would be much

more difficult that that in prokaryotes.An extensive screening process resulted to

two genera, which when challenged with aqueous metal ions such as AuCl4- and

Ag+, yielded large quantities of metal nanoparticles either extracellularly or

intracellularly. The appearance of a distinctive purple color in the biomass of

Verticillium after exposure to the 10-4 M HAuCl4 solution indicates the formation

of gold nanoparticles intracellularly and can be clearly seen in the UV-Visible

absorption spectrum recorded from the gold loaded biomass as a resonance at

~550nm. This resonance is clearly missing in the biomass before exposure to gold

ions and in the filtrate after reaction. Further evidence of the intracellullarformation is provided by a transmission electron micrograph analysis of the thin

sections of the cells after the formation of gold nanoparticles.The exposure of

Verticillium sp. To silver ions resulted to a similar intracellular growth of silver

nanoparticles. The exact mechanism leading to the intra cellular formation of

gold and silver nanoparticles by Verticillium is not fully understood. Since the

nanoparticles are formed on the surface of the mycelia and not in the solution, it

is thought that the first step involves the trapping of metal ions on the surface of

the fungal cells possibly via electrostatic interaction between the ions and thenegatively charged carboxylate groups in the enzymes present in the cell wall of

the mycelia.

Electrochemical synthesis

Maria Kulsoom

Documents

Transcript of Maria Kulsoom