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CHAPTER - II
2. Review of Literature
2.1. Green Nanoscience
Green chemistry is "the utilization of a set of principles that reduces or eliminates the
use or generation of hazardous substances in the design, manufacture and application of
chemical products". The principles of green chemistry originally defined by Anastas and
Warner have now been applied to the design of a wide range of chemical products and
processes with the aims of minimizing chemical hazards to health and the environment,
reducing waste and preventing pollution. Application of these principles has reduced the use
of hazardous reagents and solvents, improved the material and energy efficiency of chemical
processes and enhanced the design of products for end of life. Employing these principles
towards nanoscience will facilitate the production and processing of inherently safer
nanomaterials and nanostructured devices.
Green nanoscience/nanotechnology involves the application of green chemistry
principles to the design of nanoscale products, the development of nanomaterials production
methods and the application of nanomaterials [1]. The approach to develop an understanding
of the properties of nanomaterials, including those related to toxicity and to design nanoscale
materials that can be incorporated into high performance products that pose little hazard to
human health or the environment. It strives to discover synthesis/production methods that
eliminate harmful reagents and enhance the efficiency of these methods, while providing the
necessary volume of pure material in an economically viable manner. It also provides
proactive design schemes to ensure that the nanomaterials produced are inherently safer by
assessing the biological and ecological hazards in tandem with design. Finally, it seeks
applications of nanoscience that maximize societal benefit while minimizing impact on the
ecosystem. In this way, green nanoscience guides materials development, processing and
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application design throughout the life cycle, starting with raw material selection through end
of life.
Nanoparticles and other nanomaterials that exhibit size-dependent properties already
find application in products ranging from consumer healthcare goods to high performance
composites [2]. In addition, increasing number of applications of
nanoscience/nanotechnology are being developed that has promised environmental benefits
including new catalysts without distressing environment, cheap and efficient photovoltaics,
thermoelectric materials for cooling without refrigerants, lightweight (and thus energy-
conserving) nanocomposite materials for vehicles, miniaturized devices that reduce material
consumption as well as sensors that eliminate the need of unnecessary wet chemical analyses
[3-6].
2.1.1. History of nanotechnology
The end of 20th century witnessed a major scientific and technological development,
the consequences of which are only beginning to become apparent recently. There are three
factors, good understanding properties of matter at the atomic level, progress based on the
molecular approach that living organisms can also operate and the rise of information
process. These factors led to increasing unification of different disciplines of science
especially physics, chemistry and biology on the nanometer scale. Nanotechnology is an
emerging field of research and technology dealing with the fabrication and engineering of
materials, structures and systems with nanoscale size at least in one dimension [7]. The origin
of this movement is often traced to the end of 1959, the date of founding speech by Richard
Feynman ‘There is plenty of room at the bottom’ [8] made at the annual meeting of the
American Physical Society at California Institute of Technology. The term “nanotechnology”
was first coined in 1974 by Norio Taniguchi, professor of Tokyo Science University [9].
Nanotechnology is considered as an enabling technology by which existing materials,
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virtually all man-made materials and systems can acquire different properties rendering them
suitable for numerous novel applications [10]. Structural arrangement of atoms and the length
scale of the materials are two parameters, which when tailored properly at the nanometer
scale could lead to the variation in properties of materials compared to its bulk structure [11].
Figure 2.1.1 shows a picture illustrating the comparison between various naturally occurring
objects and man-made materials at different length scales.
Figure 2.1.1: Representing the relative sizes of various natural and human made objects
In order to realize practical devices with nanomaterials utilizing their unique
properties, nanoparticles with different sizes, shapes and compositions need to be
synthesized. Incidentally, significant achievements have been accomplished in this regard and
nanoparticles with myriad size and shapes over a wide range of compositions can be
synthesized today. Researchers routinely practice either “top-down or “bottom-up” approach
for the synthesis of nanoparticles. Figure 2.1.2 illustrates different structures synthesized at
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various length scales by top down and bottom up approaches. In the top-down approach
nanoparticles are synthesized by physically slicing or by abrasion of bulk material till the
desired size is achieved. In the course of evolution human being has mastered in this art by
being able to realize the structure of sub-micron level using different sophisticated techniques
such as laser induced chemical etching, ball milling etc [12].
The bottom-up approaches mainly involve chemical and biological methods to make
nanostructures and nanoparticles. These processes involve controlled condensation of solute
molecules that are formed during a chemical reaction. The restriction of the condensation or
growth leads to the formation of particles of desired size and shape [13]. However, unlike
chemical synthesis of molecules with desired structure, the synthesis of nanomaterials with
uniform size and shape is difficult. Thus, large scale synthesis of nanomaterials with specific
composition, uniform size and shape still remains an arduous task.
Figure 2.1.2: Synthesized at various length scales by top down and bottom up
approaches
Nowadays the application of nanomaterials extends to wide-range such as catalysis
[14], bio-sensing [15], drug delivery [16], diagnostics [17], solar cell [18], optoelectronics
[19], cell labelling [20], photonic band gap materials [21], single electron transistors [22],
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non-linear optical devices [23] and surface enhanced Raman spectroscopy [24]. Schematic
representations of various strategies for the synthesis of nanoparticle are given in figure 2.1.3.
Figure 2.1.3: Outline of the various approaches for synthesis of nanoparticles.
2.2. Physical synthesis methods
2.2.1. Evaporation methods
Physical vapor depositions (PVD), sputtering and chemical vapor deposition (CVD)
are the commonly used methods to form thin films of inorganic nanomaterials. PVD involves
condensation from the vapor phase. In CVD, the carrier gases containing the elements of the
desired compound flow over the surface to be coated. This surface is heated to a suitable
temperature to allow decomposition of the carrier gas and to allow the mobility of the
deposited atoms or molecules on the surface. In sputtering a discharge of non reactive ions
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such as argon is created which fall on the target and break the surface atoms, which are
collected on the surface to be coated.
2.2.2. Solvated metal atom deposition (SMAD)
In SMAD, a bulk metal is evaporated under vacuum and the vapors of the metal are
co-condensed with vapors of organic solvents like acetone to form nanoparticles in solution
using a physical method [25]. Evaporation of metal is achieved by electrically heating a metal
wire under vacuum.
2.2.3. Laser ablation
In Laser ablation technique, a bulk metal is immersed in a solvent containing
surfactant. During the laser irradiation, the metal atoms will vaporize and are immediately
solvated by the surfactant molecules to form nanoparticles in solution [26]. Metal
nanoparticles such as gold, silver and platinum nanoparticles are prepared by this way with
good control over size.
2.2.4. Photolytic and radiolytic methods
These methods involve the reduction of metal salts by radiolytically produced
reducing agents such as solvated electrons and free radicals and the photolysis of metal
complexes in the presence of some donor ligands [27]. Radiolysis of aqueous solutions of
metal ions gives solvated electrons that may directly react with the metal ions or with other
dissolved materials to produce secondary radicals, which then reduce the metal ions to form
nanoparticles.
2.3. Chemical synthesis methods
2.3.1. Sol-gel method
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The sol-gel process includes four different steps: hydrolysis, polycondensation,
drying and thermal decomposition. Metal salts or non-metal alkoxides which act as the
precursor undergo hydrolysis in presence of water or alcohols by hydrolysis. In addition to
water and alcohol, acid or base can also help to hydrolyze the precursor. This method has
been used to synthesize metal oxide nanostructures, such as TiO2, ZrO2, CeO2, SiO2, CuO,
SnO2, ZnO and Al2O3 and other nanostructures.
2.3.2. Chemical precipitation
During the synthesis of nanoparticles by chemical precipitation method, the kinetics
of nucleation and particle growth in homogeneous solutions can be adjusted by the controlled
release of anions and cations. This controlled precipitation kinetics can result in
monodisperse nanoparticles. Once the solution reaches a critical supersaturation of the
species forming particles, only one burst of nuclei occurs. Thus, it is essential to control the
factors that determine the precipitation process, such as pH and the concentration of reactants
and ions. Although this method is very simple, it is possible to prepare complicated
nanostructures (Examples CdS/HgS/CdS, CdS/(HgS)2/CdS and HgTe/CdS) [28].
2.3.3. Hydrothermal synthesis
Hydrothermal synthesis is a common method to synthesize zeolite /molecular sieve
crystals. This method exploits the solubility of almost all inorganic substances in water at
elevated temperatures and pressures and subsequent crystallization of the dissolved material
from the fluid. Water at elevated temperatures plays an essential role in the precursor material
transformation because the vapor pressure is high. The different types of oxides and sulphides
nanoparticles such as TiO2, LaCrO3, ZrO2, BaTiO3, SrTiO3, Y2Si2O7, Sb2S3, CrN, α-SnS2,
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PbS, and SnS2 nanotubes, Bi2S3
nanorods, and SiC nanowires have been successfully
synthesized.
2.3.4. Micelles or microemulsion based synthesis
In normal micelles, the hydrophobic hydrocarbon chains of the surfactants are
oriented toward the interior of the micelle and the hydrophilic groups of the surfactants are in
contact with the surrounding aqueous medium. On the other hand, reverse micelles are
formed in non aqueous medium where the hydrophilic head groups are directed toward the
core of the micelles and the hydrophobic groups are directed outward. A microemulsion is a
dispersion of fine liquid droplets of an organic compound in an aqueous solution. Such a
microemulsion system can be used for the synthesis of nanoparticles. This method is
particularly useful for the synthesis of semiconductor, oxide and metal nanoparticles such as
CdSe, CdTe, CdS, ZnS, ZrO2, TiO2, SiO2, Fe2O3, Pt, Au and Cu [29].
2.4. Biosynthesis of nanomaterials
Living organisms, especially microorganisms have a remarkable ability to form
exquisite inorganic structures often in nanodimensions. This ability of living creatures has
lured material scientists towards these biological systems to learn and improve the skills for
the precise fabrication of nanomaterials at ambient conditions. There exist several examples
in biological systems demonstrating not only the efficient synthesis of macroscopic materials
like bones and teeth with precise positioning [30] but also in making functional structures in
mesoscopic and nanometer dimensions. Generally synthesis of inorganic nanomaterials by
living creatures has been classified in two categories such as biologically controlled synthesis
and biologically induced synthesis [31]. Biologically controlled synthesis of inorganic
materials can often be considered as bio-mineralization as it is known to occur naturally in
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few specific organisms. Biogenic nanomaterials commonly have attributes which distinguish
them from their inorganic counterparts.
Unicellular organisms such as bacteria and algae are capable of synthesizing
inorganic nanomaterials, both intra and extracellularly. Biologically controlled synthesis of
inorganic materials has been studied in great detail. However it is only recently realized that
biologically induced or deliberate synthesis of nanomaterials can be an outcome of
biotechnological applications such as remediation of toxic metals occurring by reduction of
metal ions or by formation of insoluble complexes with the metal ion in the form of
nanoparticles [32]. During biologically controlled synthesis of inorganic materials, grow
within inorganic phases or organic matrix or vesicles inside the cell. It is allowing the
organism to exert a strict control over the composition, grain size, habit, and intracellular or
surface location of the produced minerals [33-35]. Examples of such synthesis include silica
biosynthesis in diatoms [36], sponges [37], radiolarians [38], calcareous structures in
coccoliths [39], gypsum in S-layer bacteria [40] and the nanocrystals of magnetite and
greigite in magnetotactic bacteria [41]. The exquisite nanostructures obtained by biologically
controlled synthesis of nanomaterials are displayed in figure 2.4.1. The confinement of
mineralization of silica and calcium carbonate leading to various exotic porous shells or
aligned structures has been explained to be directed by the geometric patterning of the
vesicles in the cells [42, 43].
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Figure 2.4.1. Images of various inorganic nanomaterials obtained by biologically
controlled synthesis. (A) diatomic silica, (B) siliceous exoskeleton of radiolarian, (C)
calcareous structures in coccolith and (D) magnetite nanocrystals from magnetotactic
bacteria.
Although, there are numerous reports on naturally occurring biological synthesis of
inorganic materials by living organisms, recently, microorganisms have been induced to
synthesize different inorganic materials such as simple metallic nanoparticles to more
complex sulphide and oxide nanoparticles. This route of biological synthesis of nanoparticles
has been realized recently, when microorganisms were used for the bioremediation of toxic
metal ions. During the deliberate or biologically induced synthesis of inorganic materials,
organisms modify its ambient microenvironment and create conditions suitable for
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extracellular precipitation of minerals. Deliberate synthesis of inorganic nanoparticles is
possible because of the specific resistant mechanism exerted by microorganisms against the
high metal ion concentration. At higher concentration of metal ions microorganisms can
cope with the toxic effect of metal ions by one of the defence mechanisms such as effluxing
of metal ions by efflux pumps, alteration in the solubility of metal ions, alteration in redox
state, extracellular compelxation and extracellular precipitation of metal ions [44].
Recently, material scientists have looked upon the detoxification of metal ions
occurring by their reduction or compelxation by microorganisms for the synthesis of
nanoparticles. Thus microorganisms can be considered as living, eco-friendly nanofactories.
Though biologically controlled mineralization or the synthesis of inorganic nanomaterials
exerts tight control over the size, shape and composition of nanoparticles it is restricted to the
synthesis of limited number of nanoparticles with different composition. On the other hand
deliberate or induced biological synthesis of inorganic nanomaterials has wide range of
composition.
In an endeavour to expand the successful demonstration of the synthesis of metal
nanoparticles based on the foundation of above mentioned observations, Sastry and co-
workers have developed a novel, botanical route for the synthesis of metallic nanoparticles.
Aqueous extracts from plants like geranium (Pelargonium graveolens), and neem
(Azadirachta Indica) have been used for the synthesis of gold, silver and gold-silver
bimetallic nanoparticles [45]. Plant based methods for the synthesis of metallic nanoparticles
can also be used for the shape directed synthesis of prism shaped gold nanoparticles. High
percentage of prism shaped gold nanoparticles could be synthesized using lemon grass
(Cymbopogan flexuosus) leaf extract [46]. It has been further demonstrated that the edge
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length and the optical properties of gold nanotriangles can be tuned by varying the
concentration of leaf extract used for the reduction of gold ions [47]. Synthesis of triangular
gold nanoparticles has also been demonstrated by using Aloe vera plant extract [48]. Liu and
co-worker have also demonstrated the synthesis of high percentage of single-crystalline
triangular gold nanoparticles using the extract of the brown sea weed Sargassum sp. [49].
Material scientists are trying to learn from nature to develop new synthetic materials with
sophisticated properties. Attempts to adopt/utilize the constructional principles of natural
materials have acquired the term bio-mimetic, the art of mimicking biology [50].
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2.5. Rambutan It is an evergreen tree growing to a height of 12-20 m. The leaves are alternate,
10-30 cm long, pinnate, with 3-11 leaflets, each leaflet 5-15 cm wide and 3-10 cm broad,
with an entire margin. The flowers are small, 2.5-5 mm, apetalous, discoidal and borne in
erect terminal panicles 15-30 cm wide. Rambutan trees are either male (producing only
staminate flowers and hence, produce no fruit), female (producing flowers that are only
functionally female), or hermaphroditic (producing flowers that are female with a small
percentage of male flowers).
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The fruit is a round to oval drupe 3–6 cm (rarely to 8 cm) tall and 3-4 cm broad, borne
in a loose pendant cluster of 10-20 together. The leathery skin is reddish (rarely orange or
yellow) and covered with fleshy pliable spines, hence the name rambutan, derived from the
Malay word rambut which means hairs.
Rambutan (Nephelium lappaceum L) fruit is consumed fresh, canned or processed and
its consumption results in the production of vast amounts of waste from seeds and peels of
the fruit. Rambutan waste such as phenolic content of its peel and seed have the antioxidant
property and antibacterial property [51]. Based on these studies we concluded that the seeds
would not provide a viable source of antioxidants, but the peel waste has significant potential
due to its powerful antioxidant properties and the large amounts of peel-materials being
generated. In plant sources, the free radicals and reactive oxygen species (ROS) are
considered to be harmful to human health and play an important causative role in disease
initiation [52, 53].
Recently, the study of the isolation of natural antioxidants from plant sources has
increased because the synthetic antioxidants, such as BHT (butylated hydroxytoluene), are
Scientific Classification
Kingdom Plantae
Order Spindales
Family Spindaceae
Genus Nephelium
Species N.lappaceum
Local Name
Rambutan
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suspected of being responsible for liver damage and carcinogenesis [54]. Plants contain a
large variety of substances possessing antioxidant activity such as vitamin C, vitamin E,
carotene, xanthophylls, tannin and phenolic [55, 56]. Vast quantities of agricultural-food
waste are produced annually worldwide. In addition the disposal of agricultural-food waste
can have a serious environmental impact. Nowadays, numerous investigations on waste
reutilization have been aimed at evaluation of the waste materials in possible value-added
applications. Agricultural waste, such as seeds and peel of grapes and/or pomegranate has
been evaluated as a rich source of natural antioxidants [57-59]. The structures of the principal
components of rambutan peel extract such as ellagic acid, corilagin and geranin are given
below.
O
O
OH
OHO
O
HO
HO
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Ellagic acid
Corilagin
Geranin
2.5.1. Uses
HO OH
OH OH HO OH
O
O O
H2C
O
O
OO
O
O
OH
OH
OH
O OHOH
OH
O O
HO
HO
O
O OH
OH
O O
HO OOH
HO
HO O H
O H O H H O O H
O
O O
H 2C
O
O
O HO H
OO H
O H
O H
O
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The main biological constituents of Rambutan are polyphenols, vitamins and amino
acids which are responsible for its versatile applications. The major constituent of Rambutan
is accumulated by polyphenols such as ellagic acid, tannins and saponins. Ellagic acid has
antiproliferative and antioxidant properties in a number of in vitro and small-animal
models. The antiproliferative properties of ellagic acid may be due to its ability to directly
inhibit the DNA binding of certain carcinogens, including nitrosamines and polycyclic
aromatic hydrocarbons. As with other polyphenol antioxidants, ellagic acid has a
chemoprotective effect in cellular models by reducing oxidative stress. It has the medicinal
application of reducing blood sugar and anticancer treatment [60].
2.6. Metal oxide nanoparticles
Nanoparticles are classified as particles having at least one dimension less than
100nm. These nanoparticles are very interesting for several reasons. They can “bridge the
gap” between bulk materials and atomic or molecular structures. This is very interesting from
a scientific point of view, because as a particle (or bulk material) gets smaller, it starts
changing its physical properties. Many sunscreens use tiny Zinc Oxide or Titanium Dioxide
nanoparticles to block the sun. Nanoparticles are able to pass through cell membranes in
biological organisms, and their effects are almost completely unknown.
2.6.1. Magnesium oxide
Magnesium oxide (MgO), or magnesia, is a white hygroscopic solid mineral that
occurs naturally as periclase and it is a source of magnesium. It has an empirical
formula of MgO and consists of a lattice of Mg2+ ions and O2− ions held together by ionic
bonds. Magnesium hydroxide forms in the presence of water (MgO + H2O → Mg (OH)2), but
it can be reversed by heating it to separate with water and MgO.
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2.6.1.1. Properties
IUPAC name Magnesium oxide
Other names Magnesia
Molecular formula MgO
Molar mass 40.30 g/mol
Appearance White powder
Odor Odorless
Density 3.58 g/cm3
Melting point 2852º C
Boiling point 3600ºC
Solubility Soluble in acid, ammonia insoluble in alcohol
Band gap 7.8 eV
Crystal structure Halite (cubic)
Coordination geometry
Octahedral (Mg2+); octahedral (O2–)
2.6.1.2. Applications
A refractory material is one that is physically and chemically stable at high
temperatures. "By far the largest consumer of magnesia worldwide is the refractory industry,
which consumed about 56% of the magnesia in the United States in 2004, the remaining 44%
being used in agricultural, chemical, construction, environmental and other industrial
applications.
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Cement: MgO is one of the raw materials for making Portland cement in dry process plants.
If too much MgO is added, the cement may become expansive. Production of MgO-based
cement using serpentinite and waste CO₂ (as opposed to conventional CaO-based cement
using fossil fuels) may reduce anthropogenic emissions of CO₂.
Desiccant: MgO is relatively poor desiccant, but because it neutralizes sulphur oxide acids
created by oxidation of Kraft-processed papers, it is used by many libraries for preserving
books.
Medical: In medicine, magnesium oxide is used for relief of heartburn and sore stomach, as
an antacid, magnesium supplement and as a short-term laxative. It is also used to improve
symptoms of indigestion. Side effects of magnesium oxide may include nausea and cramping.
In quantities sufficient to obtain a laxative effect, side effects of long-term use
include enteroliths resulting in bowel obstruction [61].
2.6.2. Copper (II) oxide
Copper (II) oxide or cupric oxide (CuO) is the higher oxide of copper. As a
mineral, it is known as tenorite. It is a black solid with an ionic structure which melts above
1200 °C with some loss of oxygen. It can be formed by heating copper in air:
2 Cu + O2 → 2 CuO
A laboratory method for preparing copper (II) oxide is to electrolyze water containing sodium
bicarbonate at a moderate voltage with a copper anode, collect the mixture of copper
hydroxide, basic copper carbonate, and copper carbonate produced, and heat it [62].
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2.6.2.1. Properties
IUPAC name Copper (II) oxide
Other names Cupric oxide
Molecular formula CuO
Molar mass 79.54 g/mol
Appearance Black to brown powder
Density 6.315 g/cm3
Melting point 1326º C
Boiling point 2000ºC
Solubility Soluble in ammonium chloride, insoluble in alcohol, ammonium hydroxide, ammonium carbonate
Band gap 1.2 eV
Refractive index 2.63
Crystal structure Monoclinic
2.6.2.2. Applications
Cupric oxide is used as a pigment in ceramics to produce blue, red, and green (and
sometimes gray, pink, or black) glazes and also copper coated on sanitary materials for
antibacterial property. It is also used to produce cuprammonium hydroxide solutions, used to
make rayon. Copper (II) oxide has application as a p-type semiconductor, because it has a
narrow band gap of 1.2 eV. It is an abrasive used to polish optical equipment. Cupric oxide
can be used to produce dry cell batteries. It has also been used in wet cell batteries as the
cathode, with lithium as an anode, and dioxalane mixed with lithium perchlorate as the
electrolyte. It is also used when welding with copper alloys.
2.6.3. Zinc oxide
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Zinc oxide is an inorganic compound with the formula ZnO. It is a white powder that
is insoluble in water, which is widely used as an additive in numerous materials and products
including plastics, ceramics, glass, cement, lubricants, paints, ointments, adhesives, sealants,
pigments, foods (source of Zn nutrient), batteries, ferrites, fire retardants and first aid tapes. It
occurs naturally as the mineral zincite but most zinc oxide is produced synthetically.
2.6.3.1. Properties
IUPAC name Zinc oxide
Other names Zinc white, Calamine, philosopher's
wool, Chinese white, flowers of
zinc
Molecular formula ZnO
Molar mass 81.40 g/mol
Appearance White solid
Density 5.606 g/cm3
Melting point 1975º C
Boiling point 2360ºC
Band gap 3.3 eV
Refractive index 2.00
Crystal structure Monoclinic
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2.6.3.2. Applications
The applications of zinc oxide powder are numerous and the principal ones are
summarized below. Most applications exploit the reactivity of the oxide as a precursor to
other zinc compounds. For material science applications, zinc oxide has high refractive index,
high thermal conductivity, binding, antibacterial and UV-protection properties. Consequently,
it is added into materials and products including plastics, ceramics, glass, cement, rubber,
lubricants, paints, ointments, adhesive, sealants, pigments, foods, batteries, ferrites, fire
retardants, etc. Some of the important are specified application are given below,
Rubber manufacture: About 50% of ZnO use is in the rubber industry. Zinc oxide along
with stearic acid is used in the vulcanization of rubber. ZnO additive also protect rubber from
fungi and UV light.
Concrete industry: Zinc oxide is widely used for concrete manufacturing. Addition of ZnO
improves the processing time and the resistance of concrete against water.
Medical: Zinc oxide as a mixture with about 0.5% iron (III) oxide (Fe2O3) is
called calamine and is used in calamine lotion. There are also two
minerals, zincite and hemimorphite, which have been historically called calamine. When
mixed with eugenol, a ligand, zinc oxide eugenol is formed, which has applications as
a restorative and prosthodontic in dentistry [63].
2.6.4. Nickel (II) oxide
Nickel (II) oxide is the chemical compound with the formula NiO. It is notable as
being the only well characterized oxide of nickel. The mineralogical form of NiO, bunsenite,
is very rare. It is classified as a basic metal oxide. Several million kilograms are produced in
varying quality annually, mainly as an intermediate in the production of nickel alloys.
28
NiO can be prepared by multiple methods. Upon heating above 400 °C, nickel powder
reacts with oxygen to give NiO. In some commercial processes, green nickel oxide is made
by heating a mixture of nickel powder and water at 1000 °C, the rate for this reaction can be
increased by the addition of NiO. The simplest and most successful method of preparation is
through pyrolysis of a nickel (II) compounds such as the hydroxide, nitrate and carbonate,
which yield a light green powder. Synthesis from the elements by heating the metal in oxygen
can yield grey to black powders which indicates nonstoichiometry.
2.6.4.1. Properties
IUPAC name Nickel (II) oxide
Other names Nickel monoxide
Oxonickel
Molecular formula NiO
Molar mass 74.69 g/mol
Appearance Green crystalline solid
Density 6.67 g/cm3
Melting point 1955º C
Solubility Soluble in ammonium hydroxide,
KCN
Refractive index 2.00
Crystal structure Monoclinic
29
2.6.4.2. Applications
NiO has a variety of specialized applications and generally applications distinguish
between "chemical", which is relatively pure material for specialty applications and
"metallurgical grade", which is mainly used for the production of alloys. It is used in the
ceramic industry to make frits, ferrites, and porcelain glazes. The sintered oxide is used to
produce nickel steel alloys. Charles Edouard Guillaume won the 1920 Nobel Prize in Physics
for his work on nickel steel alloys which he called invar and elvinar.
NiO was also a component in the Nickel-iron battery, also known as the Edison
Battery, and is a component in fuel cells. It is the precursor to many nickel salts, for use as
specialty chemicals and catalysts. More recently, NiO was used to make the NiCd
rechargeable batteries found in many electronic devices until the development of the
environmentally superior Lithium ion battery [64].
2.6.5. Applications of metal oxide nanoparticles
Nanoparticles are particles that have one dimension in 100nm or less in size and
large surface area to volume ratio. This property causes them to be more effective and applied
various fields.
Space: Nanoparticles are light weight and it is used for spacecraft and a cable for space
elevator possible. By significantly reducing the amount of rocket fuel required, these
advances could lower the cost of reaching orbit and travelling in space.
Electronics: Nanotechnologies hold some answers for how we might increase the
capabilities of electronics devices while we reduced their weight and power consumption.
Food: Nanomaterials are used to make a different not only in the taste of food safety and
health benefits that food delivers.
30
Fabric: Nanomaterials are used to fabric properties improvement without a significant
increase in weight, thickness or stiffness as might have been the case with previously used
techniques. Silver nanoparticles modified fabric that kills bacteria making clothing odor-
resistance.
Medicine: Iron oxide nanoparticles are used to improve magnetic resonance imagining scan
for cancer tumors. Quantum dots (crystalline nanoparticles) that have identify the location of
cancer cells in the body. Porous silica nanoparticles used to deliver chemotherapy drugs to
cancer cells.
Textile Industry: Metal oxide nanoparticles such as MgO, CuO, ZnO, NiO, MgO, TiO2 are
widely used for the functional finishing of cotton such as UV- protection, self-cleaning,
antimicrobial and biomedical applications [65].
Issa M El-Nahhal and its co-workers explained the synthesis, characterization and
applications of nanostructured copper oxide-cotton fibers [66]. Similarly Parthasarathi and
Thilagavathi discussed the antibacterial finish for cotton fabric from herbal products [67].
Chattopadhyay and B.H.Patel explained the improvement in physical and dyeing properties
of natural fibers through pretreatment with silver nanoparticles [68]. Asokan and its co-
workers discussed about the preparation and characterization of zinc oxide nanoparticles and
study off the anti-microbial property of cotton fabric treated with the particles [69].
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2.7. Cotton
It is a natural vegetable fibre produced in the cotton plant in many countries of the
world even in Bangladesh also. Cotton fiber is one of the oldest natural fibers which is much
familiar to human beings and widely used for numerous purposes. It is predominantly
composed of cellulose, along with hemicelluloses; there are certain non-cellulosic matters
also attached and present in the cotton fiber such as sugars, starch protein and some inorganic
matters. Cotton fibers vary in color from near white to light tan.
Cellulosic fibers exhibit good dimensional stability in the dry state but can shrink
and/or wrinkle when wet. This occurs because, in the dry state, the cellulose chains are held
together by hydrogen bonds between the hydroxyl groups of adjacent chains. In other words,
the hydrogen bonds form a crosslinked structure. If a stress, such as twisting or folding, is
applied to the dry fabric, the hydrogen bond crosslinks tend to hold the chains in position and
cause the fabric to return to their original position when the deforming stress is removed.
However, when the fabric is brought into contact with moisture, water molecules can
participate in the hydrogen bonding and penetrate between the cellulose chains, effectively
breaking up the crosslinked structure. The water molecules act as a plasticizer for cellulose
and the chains may move relative to each other. If the fabric becomes wrinkled in the moist
state, the chains move to relieve the strain and there is no effective force to return the fabric
to its original shape when the stress is removed.
2.7.1. Structure and chemical properties of cotton
The chemical constituents present in cotton are cellulose - 95%, protein - 1.3%, ash
1.2%, wax - 0.6%, sugar - 0.3%, organic acids - 0.8% and other chemical compounds.
Cellulose with the molecular formula of (C6H10O5)n is the chemical name of cotton. The
cotton fibre consists of 99% cellulose. The cellulose is a polymer consists of glucose units
32
connected with 1, 4 oxygen bridges in the β position. The -OH groups on the cellulose units
enable hydrogen bonding between 2 adjacent polymer chains. The degree of polymerization
of cotton is 9000-15000. Cellulose shows approximately 66% crystanality which can be
determined XRD and density methods. The dimensions of the cells are a = 0.835nm, b =
1.03nm and ab and bc plane is 84° for normal cellulose. Cotton fibre is composed of
concentric layers.
Figure 2.7.1: Structure of cotton
2.7.2. Advantages of cotton by human usage
1. Comfortable: Cotton fibre has large amorphous portion and hence the air can be in and
out through cotton fibre. So, the fabric made by cotton fibre is quite comfortable to use.
2. Absorbent: It has high absorbency power and this is why fibre can be dyed properly and
without any harassment.
3. Soft hand: Cotton fibre is too much regular fibre and if properly ginned; this fibre can be
the best soft hand feeling fibre amongst the others.
33
4. Hygroscopic nature: The general crispness of dry cotton textile materials may be
attributed to the rapidity with which the fibres can absorb moisture from the skin of the
fingers. This rapid absorption imparts a sensation of dryness which, in association with the
fibres inelasticity or stiffness, creates the sensation of crispness. The hygroscopic nature
ordinarily prevents cotton textile materials from developing static electricity [70, 71].
2.8. Citric acid (CA)
Citric acid is a weak organic acid. It is a natural preservative/conservative and is also
used to add an acidic, or sour, taste to foods and soft drinks. Citric acid exists in greater than
trace amounts in a variety of fruits and vegetables, most notably citrus fruits. Citric acid can
be added to ice cream as an emulsifying agent to keep fats from separating, to caramel to
prevent sucrose crystallization or to recipes in place of fresh lemon juice. Citric acid is an
excellent chelating agent and binding metals. It is used to remove lime scale from boilers and
evaporators. Citric acid is an alpha hydroxy acid and used as an active ingredient in fruit
peels.
Citric acid is preferable for starch crosslinking since low levels (5% or less) are
required for crosslinking; it can be derived from fermentation and could therefore be
considered as a green chemical. Citric acid also has price advantages over a few other
compounds commonly used to crosslink of starch. From MSDS ratings, citric acid, boric
acid, and sodium trimetaphosphate have health hazard rating of 1, whereas glutaraldehyde is
rated 2 and epichlorohydrin is rated 3. In addition to citric acid, other poly (carboxylic acids)
may also be suitable for crosslinking of starch films.
34
It is an organic acid and one of the polycarboxylic acids, which have been previously
used as environmentally friendly Durable Press finishing agent. Based on the mode of attack
on microbes, it can be considered to chemically bind to fiber surfaces and as such is a non-
leaching type. Antimicrobial activity of organic acid is attributed to pH reduction, depression
of internal pH of microbial cell by ionization of undissociated acid molecules and disruption
of substrate transport by altering cell membrane permeability or reduction of proton motive
force. The salts made in this reaction get in touch with the negatively charged protoplasm of
the microorganisms and demolish the cell membrane. This collective procedure imparts finish
that can withstand 20 wash cycles along with tumble-drying. Citric acid forms ester bonding
with the cellulose hydroxyls through the formation of anhydrides [72-74].
2.9. Sodium Alginate
It is obtained from alginic acid and also called algin or alginate. Sodium alginate is
an anionic polysaccharide distributed widely in the cell walls of brown algae, where it,
through binding water, forms a viscous gum. In extracted form it absorbs water quickly; it is
capable of absorbing 200-300 times its own weight in water. Its colour ranges from white to
yellowish-brown. It is sold in filamentous, granular or powdered forms. Alginate absorbs
water quickly, which makes it useful as an additive in dehydrated products such as slimming
aids and in the manufacture of paper and textiles. It is also used
for waterproofing and fireproofing fabrics, as a gelling agent and for thickening drinks, ice
cream and cosmetics. The chemical compound sodium alginate is the sodium salt of alginic
acid. The empirical formula is NaC6H7O6. Sodium alginate is a gum, extracted from the cell
walls of brown algae.
Among the various fibrous products, alginate-based products are currently the most
popular ones used in developing antimicrobial agents releasing systems or dressing materials.
Wang and co-worker [75] have recently demonstrated release of model drug salicylic acid
35
from alginate/polyethylene blend fibres. The frequent use of alginate based fibres has been
facilitated by its special properties such as low cost and easy availability, biocompatibility,
and ability to enhance healing of wounds, high moisture adsorption and strong ion-exchange
capacity [76]. However, in spite of possessing excellent properties for being used as dressing
material, alginate fibres cannot be used alone due to their relatively poor mechanical strength.
On the other hand, cotton fibres have been widely accepted as dressing materials due to their
fair mechanical strength, biocompatibility, durability and ease of chemical modification [77-
79].
2.10. Antibacterial activity
It is defined as to kill bacteria or to prevent the growth of bacteria. Microbes are the
tiniest creature which cannot be seen with naked eye. Microorganisms are bacteria, fungi,
algae and viruses. Bacteria are unicellular organisms which grow very rapidly under warmth
and moisture conditions. Commonly used antibacterial agent was quaternary ammonium
compounds. Organic compounds and organometallic compounds containing copper, silver,
iron and zinc resist to growth of bacteria.
Antibacterial activity is related to compounds that locally kill bacteria or slow down
their growth, without being in general toxic to surrounding tissue. Most current antibacterial
agents are chemically modified natural compounds, for instance, β-lactams (like penicillin),
cephalosporin or carbapenems. The pure natural products, such as amino glycosides, as well
as purely synthetic antibiotics, for example, sulphonamides are often used. In general, the
agents can be classified as either bactericidal, which kill bacteria, or bacteriostatic, slowing
down bacterial growth. Antibacterial agents are paramount to fight infectious diseases.
However, with their broad use and abuse, the emergence of bacterial resistance to
antibacterial drugs has become a common phenomenon, which is a major problem.
Resistance is most often based on evolutionary processes taking place during, for example,
36
antibiotic therapy and leads to inheritable resistance. In addition, horizontal gene transfer by
conjugation, transduction or transformation can be possible way for resistance to build up
[80].
2.10.1. Role of the cell wall
The bacterial cell wall is designed to provide strength, rigidity, shape and to protect
the cell from osmotic rupture and mechanical damage. According to the structure,
components and functions, the bacteria cell wall can be divided into the two main categories:
Gram positive (+) and Gram negative (–). The wall of Gram-positive cells contains a thick
layer (i.e., 20–50 nm) of peptidoglycan (PG), which is attached to teichoic acids that are
unique to the Gram-positive cell wall (Fig. 2.10.1a) [81].
By contrast, Gram-negative cell walls are more complex, both structurally and
chemically. More specifically, in Gram-negative bacteria, the cell wall comprises a thin PG
layer and contains an outer membrane, which covers the surface membrane. The outer
membrane of Gram-negative bacteria often confers resistance to hydrophobic compounds
including detergents and contains as a unique component, lipopolysaccharide, which increase
the negative charge of cell membranes and are essential for structural integrity and viability
of the bacteria (Fig. 2.10.1b)
37
Figure 2.10.1: Bacterial cell structure. (a) A Gram-positive bacterial cell wall is
composed of a thick and multilayered peptidoglycan (PG) sheath outside of the
cytoplasmic membrane. (b) A Gram-negative bacterial cell wall is composed of an outer
membrane linked by lipoproteins to thin and single-layered PG
2.10.2. Textile finishing
Recently, there has been upsurge interest in apparel technology all over the world for
much demanding functionality of the products like wrinkle resistance, water repelling, fade
resistance and resistance to microbial invasion. Among these, development of antimicrobial
textile finish is highly indispensable and relevant since garments are in direct contact with
human body [82]. Cotton fabrics provide ideal environment for microbial growth. Several
challenges have been created for apparel researchers due to increasing global demand in
textile.
Therefore, textile finishes with added value particularly for medical cloths are greatly
appreciated and there is an increasing demand on global scale. The consumers are aware of
hygienic life style and there is a necessity of textile product with antimicrobial properties.
Several antimicrobial agents’ viz., triclosan, quaternary ammonium compounds and recently
nanosilver are available for textile finishing [83]. However, cost and synthetic nature which
creates environmental problems, natural dyes in textile coloration are gaining significant
momentum. This new line of interest is due to stringent environmental standards imposed by
many countries to usage of synthetic dyes which causes allergic and toxicity. Greater interest
38
has emerged in the field of apparel technology using natural colorants, on account of their
compatibility with deodorizing properties [84].
2.10.3. Antimicrobial finish for textile materials
1. To protect the textile user against pathogenic organisms.
2. To protect wearer against odor causing microorganisms.
3. To withstand the laundry process.
4. To safe guard the textiles from damage caused by mould, mildew or rot producing
microorganisms.
5. To protect the textile products from deterioration, discoloration and staining.
6. It helps to maintain a safer and more sanitary environment
39
AIM AND SCOPE OF THE WORK
Main objective of the work is aimed for the synthesis of Magnesium Oxide, Copper
Oxide, Zinc Oxide and Nickel Oxide nanoparticles by greener approach using rambutan
peel extract.
To evaluate the antibacterial efficiency of synthesized metal oxide nanoparticles on
treatment with cotton.
The synthesized nanoparticles were characterized by the analytical techniques such as
XRD, SEM, AFM, PSA &TEM and antibacterial properties also tested. The prepared
nanoparticles were treated on cotton fabric using citric acid as well as sodium alginate
crosslinker individually to assess the effect of crosslinking agent on impregnation of
nanoparticles. The nanoparticles treated cotton fabrics were characterized by SEM with EDX
and antibacterial activity against pathogenic bacteria (S.aureus & E.coli) were analyzed by
Kirby-Bauer diffusion method.
40
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