INTRODUCTION

The dyes are
basically prepared from petroleum, in combination with mineral-derived
compounds. The first organic dye made by humans was mauveine. Today organic
dyes are widely used in textile, medicines, plastic and many other industries,
and the major concern is the hazardous effect of these dyes in water bodies. In
the water bodies amassing of organic dyes causes eutrophication, reduces
the reoxygenation capacity and does severe damage to the aquatic organisms by
hindering the infiltration of sunlight (Faisal, Abu Tariq and Muneer,
2007). Dyes are not affected by microbial action and also not
removed from different types of water treatments like ultrafilteration,
electrochemical techniques, adsorption etc. The superiority of photocatalytic
degradation by nanoparticles in wastewater treatment is due to its advantages
over the conventional methods, such as quick oxidation, no formation of
polycyclic products and oxidation of pollutants. It is an effective and rapid
technique in the removal of pollutants from wastewater (Sobana,
Muruganadham and Swaminathan, 2006). Nowadays numerous
metal nanoparticles are also used like TiO2, ZnO and other oxides.
TiO2 is of particular interest due to its stability and
low cost. Nonetheless, TiO2 has a relatively large energy band-gap and only
absorbs UV region, while the UV lights only contribute to less than 10% of
total solar radiations; the visible lights, on the other hand, contribute to
50% of the solar radiations. It’s a crucial drawback in photo-catalysis of TiO2
based application (Wu et al., 2010). To improve the
photocatalytic effect of TiO2, doping with the nitrogen and
treatment with heat is done. An alternative type of catalysts for
photodegradation of organic dyes is nanoparticles of some transition metals
such as silver. The size, shape, large surface area to volume ratio and mass
dependent reactivity has made metal nanoparticles, highly photocatalytic in
nature (Ghosh et al., 2002). For a strong future of nanotechnology, strategy of
green chemistry should be adopted for the synthesis of
nanoparticles by using renewable and eco-friendly molecules to degrade
dangerous organic solvents and chemical reducing agents. Colloid based
nanotechnology has been developed to control the size, shape, uniformity and
functionality. The biosynthesized AgNPs from plant source exhibits high
degradation action under a source of visible light.

PROPERTIES OF NANOPARTICLES

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The size of
particle has direct influence on the chemical and physical properties of the
particle. When the particle size is in the nano scale, then it behaves totally
different from their bulk counterpart.

·        
When the particle size
is in nano range, then surface molecules is higher in number and hence the
greater number of dangling bonds. The greater is dangling bonds higher the
surface free energy. Therefore, it is energetically favorable for stable
structure to lower the number of dangling bonds and thereby reducing the
surface free energy.

·        
Nanoparticles behave
like an individual atoms due to the formation of discrete energy level rather
than the continuum energy levels.

·        
The reduction in
particle size from bulk to nano range results in an increase in proportion of
surface energy and also the reduction in internal spacing.

·        
There is a decrease in
the melting point with respect to decrease in particle size.

·        
Optical properties vary
with the change in size of the component.

·        
Reduction in size of
particle leads to the increase in the band gap which results in  the shift of light absorption towards the
high energy region (blue shift) (Viswanathan, 2009).

·        
The energy gap also
increases when the particle size reduces, and hence the conductivity reduces.

·        
Nanocrystalline
ceramics are soft and have lower temperatures.

·        
Reduction of material
at nano scale can increase its fatigue strength upto 250%.

·        
After a reduction in
particle size, there is an increase in surface area and the availability of the
reactant molecules increased, therefore increase in catalytic activity.

PRODUCTION OF
NANOPARTICLES BY PLANTS

The consumption
of plants as the production of silver nanoparticles has drawn attention,
because of non-pathogenic, eco-friendly, economical protocol and providing a
single step technique for the biosynthetic processes. Quantum dots are
semiconductor molecules, which have size of nanometers. They are so small that
their properties differ from its large counterpart. These can be easily produced
by the plants like Alfalfa (Mohanpuria, Rana and Yadav, 2007).

The protocol
for the synthesis of nanoparticle involves: the collection of plant part and
washed with sterile water thoroughly for 2-3 times to remove both epiphytes and
necrotic plants; then clean plant part shade-dried for 10-15 days and finely
powdered. For the plant broth preparation, around 10g of dried powder is boiled
with 100ml distilled water. The resulting solution is filtered thoroughly until
no insoluble material observed in the broth.

To the plant
extract 10-3 M of AgNO3 solution added drop wise,
addition of few ml of plant extract follow the reduction of pure Ag (I) ions to
Ag (0). The end point is change in color of plant extract from green to yellow
or dark brown. The Erlenmeyer flasks were incubated at 37 °C under agitation
(200 rpm) for 24–48 h. Then centrifuged at 4000 rpm for 30min at 4o C,
pellet was obtained and resuspended in the distilled water. It was repeated 2-3
times until complete purification of AgNPs. The solution is then lyophilized
and after freeze drying powdered AgNPs were obtained.

The purity of the production of silver nanoparticles can
be monitored by measuring the UV–visible spectra of the solution at regular intervals
(Ahmed et al., 2015). Various other methods can also be used to characterize
the silver nanoparticles, including SEM, TEM, FTIR, XRD.

The relatively high levels of the steroids,
carbohydrates, sapogenins and flavonoids act as reducing agents and
phyto-constituents as the capping agents which provide stability to silver
nanoparticles. The synthesized nanoparticles found to be of average size around
7–17 nm and are of spherical shaped. These nanoparticles were found to
have a crystalline structure with face cantered cubic geometry. With different
plant samples, different size of nanoparticles can be formed. The nanoparticles
produced by plants are more stable than those produced by other organisms, as
plant reduces the metal ions faster than bacteria or fungi.

CHARACTERIZATION OF
NANOPARTICLES AND ASSOCIATED MOLECULES

Microscopic techniques such as scanning electron
microscopy, transmission electron microscopy and atomic force microscopy are
mainly used for morphological studies of nanoparticles. The formation of
different nanoparticles shows different peaks that can be characterized by
UV-Vis spectroscopy. The absorption peak for silver nanoparticles formation by
the silver ions is approximately 450 nm. The continuous increase in the peak
with the increase in plant extract concentration with salt ions and the
reaction time, clearly indicates the formation of nanoparticles.

The X-Ray diffraction (XRD) is used to characterize the
metallic nature of particles. As the wavelength of X-rays is comparable to the
size of atoms hence is used to determine its structural arrangements. X-rays
penetrate deep into the material therefore, provides the information about the
bulk structure (Kumar and Yadav, 2008).

FTIR (Fourier transform infrared) spectroscopy is an
analytical technique that measures the wavenumber of light i.e. infrared
intensity versus wavelength. It determines the property of relation between
plant extracts and nanoparticles. Vibration characteristics of functional group
in sample are detected by infrared spectroscopy. Infrared light absorbed by
chemical functional groups in a specific wavenumber range irrespective of the
structure of the rest of the molecule. Hence, the correlation of band
wavenumber position with chemical structure is used to identify a functional
group in the nanoparticle associated molecule in a sample.

Raman spectroscopy is mainly used to determine the sample
temperature, material composition, etc. In this technique small amount of the
sample is required and the non-destructive optical spectrum is effortlessly
achieved. The optical properties of the nanoparticles are important as the
biological, cell imaging and photothermal therapeutic applications grounded
upon the optical characteristics (Kumar and Yadav, 2008).

NANOMATERIALS IN
PHOTOCATALYSIS

To degrade the pollutants in an environment different
methods are adopted, photocatalysis is one of the attention gaining technique.
The aqueous system of nature is mostly purified by sunlight, that initiate the
breakdown of organic compounds into simpler forms, carbon dioxide and other
mineral acids. As the degradation of  organic
compounds does not produce any toxic compounds, therefore it is the  major advantage of the photocatalytic process
over existing technologies and  there is also
no further requirement for secondary disposal methods (Beydoun et al.,
1999). In photocatalysis expensive oxidizing agents are not required as
ambient oxygen acts as oxidant. An ideal photocatalyst should be
stable, non-toxic, inexpensive and, highly photoactive. Disadvantage of using
the nanoparticles, that they require light of shorter wavelength for
photocatalysis.

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