32 CHAPTER 2 REVIEW OF LITERATURE This review attempts to explain the diversity of the field, starting with the nanotechnology and its applications, various methods of synthesis of nanoparticles and the possible mechanistic aspects. The next section discusses about the various dyes used in textiles and the methods adopted for removal/biosorption of textile dyes. The last section highlights the recent advances, possible applications of nanoparticles in photocatalytic studies with the future perspectives. Though there are a few good reviews dealing with the synthesis and applications of nanoparticles, there appears to be scarcity of information regarding the possible mechanistic aspects of nanoparticle formation. This review attempts to fill the void. 2.1 NANOTECHNOLOGY The concept of nanotechnology though considered to be a modern science has its history dating back from 9th century. Nanoparticles of gold and silver were used by the artisans of Mesopotamia to generate a glittering effect to pots. The first scientific description of the properties of nanoparticles was relations of gold (and o 1857). In 1959, Richard Feynman gave a talk describing molecular machines built with atomic precision. This was considered the first talk on nanotechnology.
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32
CHAPTER 2
REVIEW OF LITERATURE
This review attempts to explain the diversity of the field, starting with
the nanotechnology and its applications, various methods of synthesis of
nanoparticles and the possible mechanistic aspects. The next section discusses
about the various dyes used in textiles and the methods adopted for
removal/biosorption of textile dyes. The last section highlights the recent
advances, possible applications of nanoparticles in photocatalytic studies with the
future perspectives. Though there are a few good reviews dealing with the
synthesis and applications of nanoparticles, there appears to be scarcity of
information regarding the possible mechanistic aspects of nanoparticle formation.
This review attempts to fill the void.
2.1 NANOTECHNOLOGY
The concept of nanotechnology though considered to be a modern
science has its history dating back from 9th century. Nanoparticles of gold and
silver were used by the artisans of Mesopotamia to generate a glittering effect to
pots. The first scientific description of the properties of nanoparticles was
relations of gold (and o 1857).
In 1959, Richard Feynman gave a talk describing molecular machines
built with atomic precision. This was considered the first talk on nanotechnology.
36
and the amount of biological material has come up to give hope in
implementation of these approaches in large scale and for commercial
applications. There are also the possibilities of producing genetically engineered
size and shape of biological nanoparticles. The combinatorial approach such as
photobiological methods as proved in the case of Fusarium oxysporum-mediated
silver nanoparticles production will help to increase the rate of production. While
exploring the natural secrets for the synthesis of nanoparticles by microbes, which
are regarded as potent ecofriendly green nanofactories, scientists have discovered
magnetite particles by magnetotactic bacteria (Lovley et al 1987; Dickson 1999),
siliceous materials by diatoms (Pum & Sleytr 1999), and gypsum and calcium
layers by S-layer bacteria (Milligan 2002). Interactions between metals and
microbes have been exploited f
bioremediation, biomineralization, bioleaching, and biocorrosion and the
research as nanobiotechnology inter connecting biotechnology and
nanotechnology.
2.1.2.1 Properties of nanoparticles
A number of physical phenomena become more pronounced as the size
of the system decreases. Certain phenomena may not come into play as the
system moves from macro to micro level but may be significant at the nano scale.
One example is the increase in surface area to volume ratio which alters the
mechanical, thermal and catalytic properties of the material. The increase in
surface area to volume ratio leads to increasing dominance of the behaviour of
atoms on the surface of the particle over that of those in the interior of the particle,
thus altering the properties. The electronic and optical properties and the chemical
reactivity of small clusters are completely different from the better known
property of each component in the bulk or at extended surfaces. Some of the size
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dependant properties of nanoparticles are quantum confinement in
semiconductors, Surface Plasmon Resonance in some metallic nanoparticles and
paramagnetism in magnetic nanoparticles.
Surface plasmon resonance refers to the collective oscillations of the
conduction electrons in resonance with the light field. The surface plasmon mode
arises from the electron confinement in the nanoparticle. The surface plasmon
resonance frequency depends not only on the metal, but also on the shape and size
of the nanoparticle and the dielectric properties of the surrounding medium (Jain
et al 2007). For example, noble metals, especially gold and silver nanoparticles
exhibit unique and tunable optical properties on account of their Surface Plasmon
Resonance.
Super paramagnetism is a form of magnetism that is a special
characteristic of small ferromagnetic or ferromagnetic nanoparticles. In such
super paramagnetic nanoparticles, magnetization can randomly change direction
under the influence of temperature. Super paramagnetism occurs when a material
is composed of very small particles with a size range of 1- 10nm. In the presence
of an external magnetic field, the material behaves in a manner similar to
paramagnetism with an exception that the magnetic moment of the entire material
tends to align with the external magnetic field.
Quantum confinement occurs when one or more dimensions of the
nanoparticle is made very small so that it approaches the size of an exciton in the
bulk material called the Bohr exciton radius. The idea behind confinement is to
trap electrons and holes within a small area (which may be smaller than 30nm).
Quantum confinement is important as it leads to new electronic properties.
Scientists at the Washington University have studied the electronic and optical
changes in the material when it is 10nm or less and have related it to the property
of quantum confinement. Some of the examples of special properties that
nanoparticles exhibit when compared to the bulk are the lack of malleability and
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ductility of copper nanoparticles lesser than 50nm. Zinc oxide nanoparticles are
known to have superior UV blocking properties compared to the bulk.
2.1.2.2 Kinds of Nanoparticles
Nanoparticles can be broadly grouped into two: namely organic and
inorganic nanoparticles. Organic nanoparticles may include carbon nanoparticles
(fullerenes) while some of the inorganic nanoparticles may include magnetic
nanoparticles, noble metal nanoparticles (like gold and silver) and semiconductor
nanoparticles (like titanium dioxide and zinc oxide). There is a growing interest
in inorganic nanoparticles as they provide superior material properties with
functional versatility.
Due to their size features and advantages over available chemical
imaging drugs agents and drugs, inorganic nanoparticles have been examined as
potential tools for medical imaging as well as for treating diseases. Inorganic
nanomaterials have been widely used for cellular delivery due to their versatile
features like wide availability, rich functionality, good biocompatibility, and
capability of targeted drug delivery and controlled release of drugs (Xu et al
2006). For example mesoporous silica when combined with molecular machines
prove to be excellent imaging and drug releasing systems.
Gold nanoparticles have been used extensively in imaging, as drug
carriers and in thermo therapy of biological targets. Inorganic nanoparticles (such
as metallic and semiconductor nanoparticles) exhibit intrinsic optical properties
which may enhance the transparency of polymer- particle composites. For such
reasons, inorganic nanoparticles have found special interest in studies devoted
to optical properties in composites. For instance, size dependant colour of gold
nanoparticles has been used to colour glass for centuries.
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2.2 SILVER NANOPARTICLES
Silver nanoparticles are nanoparticles of silver, i.e. silver particles of
between 1 nm and 100 nm in size. While frequently described as being 'silver'
some are composed of a large percentage of silver oxide due to their large ratio of
surface-to-bulk silver atoms. Over the last decades silver nanoparticles have
found applications in catalysis, optics, electronics and other areas due to their
unique size-dependent optical, electrical and magnetic properties. Currently most
of the applications of silver nanoparticles are in antibacterial/antifungal agents in
biotechnology and bioengineering, textile engineering, water treatment, and
silver-based consumer products. Silver nanoparticles are commonly utilized
nanomaterials due to specific surface area is relevant for catalytic reactivity,
high electrical conductivity, and unique optical properties that could be used in
various applications (Sondia & Salopek- Sondi 2004). Metal nanoparticles are
synthesized and stabilized through chemical and mechanical methods
(Balantrapu & Goia 2009; Tripathi et al 2010), electrochemical techniques
(Patakfalvi & Dekany 2010), photochemical reactions in reverse micelles
(Rodriguez-Sanchez et al 2000) and nowadays via green chemistry method
(Taleb et al 1998).
2.2.1 Green Synthesis of Silver Nanoparticles
The worldwide demand for environmentally friendly and sustainable
methods to prepare nanomaterials requires the application of green chemistry
principles. Thus green nanoscience aims at using environmentally benign and
economically viable reagents or solvents, designing inherently safe
nanomaterials for reduced biological and ecological detriment, and enhancing
the material and energy efficiency of safe chemical processes (Dahl et al
2007). Nanotechnology is an upcoming field the benefits of which are
believed to revolutionize various other fields like computers, pharmaceuticals
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etc. The synthesis of nanoparticles of different chemical composition, controlled
size is an important area of research in nanotechnology. Nanoparticles of metals
and semiconductors have immense use in various other branches of
sciences. There are various chemical methods (Murray et al 2002) and
physical methods (Ayyub et al 2001) to synthesize nanoparticles, but these routes
for synthesis of particles/crystallites require tedious and environmentally
challenging techniques.
The growing needs to develop clean, non-toxic and eco-friendly
procedures for synthesis of nanoparticles has resulted in researchers seriously
looking at biological systems for inspiration. Ever increasing pressure to
develop environmentally benign technique for nanoparticle synthesis has lead
to a renewed interest in biotransformation as a route to growth of nanoscale
structures. Biological systems have a unique ability to control the structure,
phase and nanostructural topography of the inorganic crystals (Cui & Gao
2003). It is well known that microbes such as bacteria (Brierley 1990), Yeast
(Huang et al 1990), fungi (Frilis & Myers-Keith 1986) and algae are able to
adsorb and accumulate metals and can be used in the reduction of
environmental pollution and also for the recovery of metals from waste.
Amongst these microorganisms, only a few groups have been confirmed to
selectively reduce certain metal ions (Klaus et al 1999, Mukherjee et al 2001;
Nair & Pradeep 2002; Oremland et al 2004). The potential of microbes to
Semiconductor bimetallic nanoparticles with immense use in the
semiconductor devices.
Biosynthetic methods can be employed either microorganism cells
or plant extract for nanoparticles production. Biosynthesis of nanoparticles is
an exciting recent addition to the large repertoire of nanoparticles synthesis
methods and now, nanoparticle has entered a commercial exploration period.
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Gold and silver nanoparticles are presently under intensive study for applications
in optoelectronic devices, ultrasensitive chemical and biological sensors and as
catalysts. This chapter is devoted to biosynthesis and application of silver
nanoparticles.
Nanotechnology enables the development of nanoscale particles of
metals with novel and distinctive physic-chemical properties, and a broad range
of scientific and technological applications (Moore 2006). Another potential use
of silver nanoparticles in water filters in wastewater treatment plants. At
nanoscale silver exhibits remarkably unusual physical, chemical and biological
properties (Evan off & Chumanov 2005; Chen & Schluesener 2007). Recently it
was shown that silver ions may be reduced extracellularly using fungus
Phanerochaete chrysoporium (Vigneshwaran et al 2007) and Pleurotus sajor caju
(Nithya & Ragunathan 2009).
Silver nanoparticles are the promising products in the
nanotechnology industry. The development of consistent processes for the
synthesis of silver nanomaterials is an important aspect of current
nanotechnology research. One of such promising process is green synthesis.
Silver nanoparticles can be synthesized by several physical, chemical and
biological methods. However for the past few years, various rapid chemical
methods have been replaced by green synthesis because of avoiding toxicity of
the process and increased quality. Synthesis of nanoparticles to have a better
control over particles size, distribution, morphology, purity, quantity and
quality, by employing environment friendly economical processes has always
been a challenge for the researchers (Hahn 1997). The synthesis and assembly
of nanoparticles would benefit from the development of clean, nontoxic and
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organisms ranging from bacteria to fungi and even plants (Sastry
et al 2004).
2.2.2 Significance of Silver Nanoparticles
The preparation of stable, uniform silver nanoparticles by reduction of
silver ions by ethanol is reported here. It is a simple process of recent interest for
obtaining silver nanoparticles. The samples have been characterized by X-Ray
diffraction (XRD) and Transmission Electron Microscopy (TEM), which reveal
the nano nature of the particles. These studies infer that the particles are mostly
spherical in shape and have an average size of 16 nm. The UV/Vis spectra show
that an absorption peak, occurring due to Surface Plasmon Resonance (SPR),
exists at 410 nm.
Uniform silver nano particles can be obtained through the reduction of
silver ions by ethanol at the temperature of 80°C to 100°C under atmospheric
conditions. In this synthesis process, 20 ml of aqueous solution containing silver
nitrate (0.5g of AgNO3), 1.5 g sodium linoleate (C18H32ONa), 8 ml ethanol and 2
ml linoleic acid (C18H32O2) are added in a capped tube under agitation. The
system is sealed and treated at the temperatures between 80°C to 100°C for 6
hours.
In the aqueous solution of silver ions, sodium linoleate and the mixture
of linoleic acid and ethanol are added in order. A solid phase of sodium linoleate,
a liquid phase of ethanol and linoleic acid, and water ethanol solution phase
containing silver ions formed in the system. Ethanol in the liquid and solution
phases reduced the silver ions into silver nanoparticles. Along with the reduction
process, linoleic acid is absorbed on the surface of the silver nanoparticles with
the alkyl chains on the outside which the produced silver nanoparticles of near
circular shape.
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The product which collected at the bottom of vessel after cooling to
room temperature was dispersed in chloroform to form a homogenous colloidal
solution of silver nanoparticles. The colour of the sample (colloidal solution of
silver nanoparticle) becomes reddish brown. On changing the concentration of
electrolyte, it is found that the colour become reddish brown on adding linoleic
acid at the same proportions. This reddish brown colour of prepared nanoparticles
indicates nearly 100% conversions of silver ions into nanoparticles. The
preparation of silver nanoparticles with different electrolyte concentrations has
been tried, but neither the samples with concentration other than the present one is
found to be stable over 2 weeks nor of smaller size (more than 60 nm). Hence, we
have recorded the data of the particle of optimum size and of comparatively better
stability (over 4 months).
Silver nanoparticles are being used in numerous technologies and
incorporated into a wide array of consumer products that take advantage of their
desirable optical, conductive, and antibacterial properties. Silver nanoparticles
have advantages over other noble nanoparticles (e.g., gold and copper) because
the surface Plasmon resonance energy of silver is situated far from the
interband transition energy. Application of green chemistry to the synthesis of
nanomaterials has vital importance in medicinal and technological aspects
(Mondal et al 2011; Begum et al 2009). Biologically synthesized silver
nanoparticles (Ag-NPs) have wide range of applications because of their
remarkable physical and chemical properties. The literature on the extra cellular
biosynthesis of Ag-NPs using plants and pure compounds from plants are
insignificant (Song & Kim 2008; Gilaki 2010). Specifically, while there is
relatively little or no literature on the extra cellular synthesis of Ag-NPs by using
seaweeds (Govindaraju et al 2009). The following are some of the specific
applications of silver nanoparticles:
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Diagnostic Applications: Silver nanoparticles are used in
biosensors and numerous assays where the silver nanoparticle
materials can be used as biological tags for quantitative detection.
Antibacterial Applications: Silver nanoparticles are
incorporated in apparel, footwear, paints, wound dressings,
appliances, cosmetics, and plastics for their antibacterial
properties.
Conductive Applications: Silver nanoparticles are used in
conductive inks and integrated into composites to enhance
thermal and electrical conductivity.
Optical Applications: Silver nanoparticles are used to efficiently
harvest light and for enhanced optical spectroscopies including
metal-enhanced fluorescence (MEF) and surface-enhanced
Raman scattering (SERS).
Environmental applications: Silver nanoparticles are used to
provide an efficient degradation of various pollutants from
industries.
2.3 SEAWEEDS AS A SOURCE OF SYNTHESIS OF SILVER
NANOPARTICLES
Earth is a biosphere sizzling with activities of all terms of life. It is
essentially a water planet, two thirds of which being covered with water
especially marine water. Seaweeds or marine microalgae are the renewable
living resources which are also used as food, feed and fertilizer in many parts of
the world. Seaweeds are of nutritional interest as they contain low calorie food,
but rich in vitamins, minerals and dietary fibres. In addition to vitamins and
minerals, seaweeds are also potentially good sources of proteins, polysaccharides
45
and fibres (Darcy-Vrillon1993). The lipids which are present in very small
amounts are unsaturated and afford protection against cardiovascular pathogens.
Seaweeds are considered as source of bioactive compounds and produce a great
variety of secondary metabolites characterized by a broad spectrum of biological
activities. Compounds with cytostatic, antiviral, antihelminthic, antifungal and
antibacterial activities have been detected in green, brown and red algae. Many
bioactive compounds can be extracted from seaweeds. Seaweeds have been
screened extensively to isolate life saving drugs or biologically active substances
all over the world. The present study was undertaken to investigate the
biosorption of textile dyes.
Seaweeds are rich and varied source of bioactive natural products and
have been studied as potential biocidal and pharmaceutical agents (Ara et al
2001). In recent years, there are numerous reports of macro algae derived
compounds that have a broad range of biological activities such as antibacterial,
antifungal, antiviral, antineoplastic, antifouling, anti inflammatory, antitumoric,
cytotoxic and anti-mitotic activities (Perry et al 1991). Presently seaweeds
constitute commercially important marine renewable resources which are
providing valuable ideas for the development of new drugs against cancer,
microbial infections and inflammations.
Seaweeds or benthic marine algae are the group of plants that live
either in marine or brackish water environment. The synthesis of nanoparticles
using algae as source has been unexplored and underexploited. More recently,
there are few, reported that algae being used as a biofactory for synthesis of
metallic nanoparticles.They are also rich in polysaccharides such as alginates,
fructans, and laminarians which have been reported to possess potential medicinal
46
a. Seaweeds are divided into three categories depending upon their
nutritional and chemical composition as brown, red and green algae.
Brown seaweeds are known to contain more bioactive components
than either green or red seaweeds. Marine algae in human consumption have been
documented since 600 BC. In China, Japan, France and Chile, seaweeds are
harvested to be included in a great variety of dishes, including sushi wrappings,
salads, soups, and condiments. The major seaweeds that are of dietary importance
in Asian countries belong to the genus Undari, Porphyra and Laminaria. Recently,
other countries, such as USA, South America, Ireland, Iceland, and France have
market in seaweeds
(McHugh 2003). The Atlantic coast of Ireland is one of the most productive
seaweed growing areas in the world. Its climate, over 5000 km of rocky habitat,
seaweed grow. More than 600 di erent species of seaweed have been
Irish waters. How
of this product from coastal regions and it may be collected from inshore marine
waters as well as from agricultural run-o which may perhaps be a source of
contamination. The retail of fresh seaweeds is limited and the harvest is processed
in order to ensure preservation by drying, freezing or fermenting. Not much
literature is available investigating the micro biology of this material.
Hence, the present study was performed to characterize the brown
seaweeds with respect to their biodiversity and the e ects of heat on its
microbiota.
2.3.1 Ulva lactuca
Ulva lactuca Linnaeus, a green alga in the division Chlorophyta, is the
type species of the genus Ulva, also known by the common name sea lettuce.
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Ulva lactuca is a thin flat green algae growing from a discoid holdfast. The
margin is somewhat ruffled and often torn. It may reach 18 cm or more in length,
though generally much less, and up to 30 cm across. The membrane is two cells
thick, soft and translucent, and grows attached, without a stipe, to rock by a small
disc-shaped holdfast. Green to dark green in color, this species in the Chlorophyta
is formed of two layers of cells irregularly arranged, as seen in cross section. The
chloroplast is cup-shaped with 1 to 3 pyrenoids. There are other species of Ulva
which are similar and not always easy to differentiate.
2.4 TEXTILE INDUSTRY
Environmental pollution has increased with increasing industrial
developments (Noor-ul-Amin et al 2009). In recent years, globalization and
rapid industrialization have resulted in the establishment of variety of industrial
sectors. Among which, the invention of synthetic dyestuffs has enhanced the
growth rate of new textile sectors in the developing countries like India. Cheap
labor availability also has become a support to the growth of such textile sectors.
There are more than 1, 00,000 commercially available dye industries with over
approximately 7 × 105 tonnes of dyes utilized annually through out the world
(Nigam et al 1996). Thousands of small-scale dyeing units generate enormous
amount of wastewaters from dyeing and subsequent rinsing steps that form one of
the largest contributions to wastewater generation in the textile industry. Textile
dyeing and processing industries use variety of synthetic dyestuffs such as acid,
base, vat, sulphur, indigo, azo and reactive dyes depending on the nature of the
fabric and the intensity of color required thus leads to generation of various types
of wastewater differing in magnitude and quality. The main challenge for the
textile industry today is to modify production methods, so they are more
ecofriendly at a competitive price, by using safer dyes and chemicals and by
reducing cost of effluent treatment/disposal. Recycling has become a
48
necessary element, not because of the shortage of any item, but because of the
need to control pollution.
2.4.1 Wastewater from Textile Industry
Wastewater generated from different industries is posing a great
threat not only to mankind but also to the landmass fertility as well as natural
flora and fauna. In order to meet the stringent international standards,
treatment of industrial wastewater is mandatory. Different dyes and pigments
are extensively used in textiles, papers, plastics, cosmetics, pharmaceuticals
and food industries (Levin et al 2005). The total world colorants production is
estimated to be 8, 00,000 tons per year and at least 10 % of the used dyestuffs
enter the environment through textile and other mills effluents (Levin et al
2004; Palmieri et al 2005). The wastewater from textile plant is often rich in
color, containing residual dyes and chemicals. The presence of dyes in the effluent
can have acute and/or chronic effects on exposed organisms or living beings
depending on the exposure time and dye concentration. Increased color
concentrations not only make the water unfit for domestic or industrial uses, but
they also reduce light transmittance, thus limiting aquatic plant growth and self-
purification processes. In addition, dyes exert an adverse effect on fish life.
Currently the effluents are discharged into streams or canals after retention period
of few hours in a stabilization pond without any proper treatment. This effluent
has adverse impacts on under ground, surface water bodies and land in the
surrounding area. The ecological system of the neighborhood is badly affected
and hence proper treatment method is required before releasing into the
environment.
Till recently, the discharging of waste into the environment was the
way to eliminate them. The permitted discharge levels have been vastly exceeded,
causing such environmental contamination that our natural resources cannot be
49
used for certain purposes and their characteristics have been altered. Dyes,
phenols, pesticides, fertilizers, detergents, and other chemical products are
disposed directly into the environment, without being treated, controlled or
uncontrolled and without an effective treatment strategy. Color
removal/adsorption from the textile wastewater has become an issue of interest
during the last few years because of the toxicity of the dyes and more often the
colored wastewater from the textile industries also decreases the visibility of the
receiving waters. Removal of dyes from textile waste effluents has been carried
out by physical, chemical and biological methods, such as flocculation,