APPLICATIONS OF ZINC OXIDE NANOSTRUCTURES (ZnO-Ns)...
Transcript of APPLICATIONS OF ZINC OXIDE NANOSTRUCTURES (ZnO-Ns)...
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CHAPTER 5
APPLICATIONS OF ZINC OXIDE NANOSTRUCTURES
(ZnO-Ns)
5.1 Introduction
The nanoparticle synthesis of controlled size, its distribution, shape and
surface state is recognized to be of prime importance as their properties are
essential for successful application. Among all nanomaterials, nanoparticles
of metal oxides are very attractive as their unique characteristics make them
the most diverse class of materials with properties covering almost all aspects
of solid-state physics, materials science and catalysis. Indeed, the crystal
chemistry of metal oxides, i.e. the nature of the bonding, varies from highly
ionic to covalent or metallic.
Nanoscale Zinc oxide 1-6 is an inorganic compound appearing invisible
when added to other materials like ointments and fungal powders. Its
transition from micro to nano form increases the material surface and hence
improves different properties. Zinc Oxide have found a good citation since
few decades and found a broad spectrum for applications as cold, rashes,
sunscreen lotions; in paints and coatings; in LED’s, transparent thin film
coatings and various others, because of their various characteristic properties
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evolved at nano stage. A colloidal system (solid, liquid or gas) consists of two
different phases namely a dispersed phase (or internal phase) with particle
diameter of 5 – 200 nm and a continuous phase (or dispersion phase).
Colloidal science plays a vital role in the field of biological science along
with our environment. The Nanoscale Zinc Oxide colloidal science has
opened an important paradigm for study as far as its anti-bacterial, anti-
fungal, photo-catalytic activity and UV filtering properties are concerned. The
anti-microbial effect and photo-catalytic degradation by Zinc oxide
nanostructures have been presented below:
5.2 Anti-microbial Activity of ZnO Nanostructures (Ns)
5.2.1 Introduction
Due to diversified uses of Metal Oxide Nanoparticles
(MONPs) in research, industrial and health related applications,
metal oxide nanoparticles are increasingly being developed
through inexpensive and user friendly approaches. Hence, in other
words, the effect of MONPs with the pathogenic microbes is an
evolving field of research. Out of these, Zinc oxide Nanoparticles
(ZnO-Nps) are useful as antimicrobial agents against microbes of
therapeutic significance when blended with medicines, ointments
and personal care products. Due to specific compatibility towards
both aqueous and organic solvents, ZnO-Nps are allowing for
incorporation into most material processes, therefore, ZnO-Nps are also
considered as bacteriostatic and fungistatic against microbes of
industrial importance when included into materials, such as surface
coatings (paints), wallpapers, textile fibers, plastics and ceramics. The
advertisements seen by us for antibacterial refrigerators and bathroom
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tiles are some of the best examples. There are several examples of
highly significant antimicrobial activity of ZnO-Nps against medically
and industrially important Gram-positive and Gram-negative
bacteria, ascomycetes and protozoans 7-20.
The antimicrobial potentiality of ZnO-Nps against different
disease causing pathogenic microbial strains has been already
evaluated using qualitative and quantitative assays. The applications
of ZnO-Nps for Staphylococcus aureus (cellulitis) 33 [Seil et al.,
2011], Salmonella typhimurium (typhoid fever) 22 [Kumar et
al., 2011], Candida albicans (candidiasis) 17 [Lipovsky
et al., 2011], Listeria monocytogenes (septicaemia)
30 [Arabi et al., 2012], Campylobacter jejuni (Guillain-
Barré syndrome) 25 [Xie et al., 2011] and Pseudomonas aeruginosa
(bacteremia) 21 [Feris et al., 2010] are well documented. Besides,
ZnO-Nps could potentially be used as an effective antimicrobial agent
to protect agricultural and food safety from foodborne
pathogens especially Bacillus subtilis, Escherichia coli,
Pseudomonas fluorescens 31 [Jiang et al., 2009], Salmonella
enteritidis 32 [Jin et al., 2009] and Botrytis cinerea, Penicillium
expansum 19 [He et al., 2011] by disintegrating the cell membrane
and increasing the membrane permeability and oxidative stress to
lyse these food-borne bacteria 25,32 [Jin et al., 2009; Xie et al., 2011].
These points suggested that the application of ZnO-Nps as
antimicrobial agents in medicine and food systems may be
effective for microbial growth inhibition.
Plausibly, the antibacterial activity of ZnO-Nps is dependent on
its size, morphology and architecture of nanoparticulate 34
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[Yamamoto, 2001]. The enhanced surface area of ZnO-Nps allows
for increased interaction with microbes which permits using a smaller
amount of ZnO-Nps for the same or improved biostatic behavior.
Moreover, the microbicidal property of these nanoparticles
suggested that these particles were more diffusible in the growth
medium which in turn allowed greater interaction between microbial
cells and nanoparticle. From the previous reports, it is considered
that ZnO-Nps-cell surface interaction affects the cell morphology as
well as cell membrane permeability, consequently the entry
of ZnO-Nps induces oxidative stress in bacterial cells resulting
in the inhibition of cell growth and eventually cell death.
All of the above information regarding ZnO-Nps suggests that
these nanoparticulates have a potential application as a pathogen
growth inhibitor and may have future applications in the development
of derivative agents to control the spread and infection of a variety
of microbial strains.
From the literature, the studies revealed that the nanoscaled ZnO
particles were more toxic to all bacterial species than other
nanoparticles of different inorganic oxides like aluminium oxide,
silicon dioxide, etc 21-30.
The Gram +ve bacteria, Bacillus subtilis is a rod shaped bacteria
which is commonly known as normal gut commensal in humans.
Due to the emergence of anti-biotic resistant drugs, alternate
medications are under primary considerations. A noteworthy
experimentation was concerned with anti-bacterial activity of
therapeutically viable Gram +ve bacteria, Bacillus subtilis and
it was found that reported ZnO-NBs have become the
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promising entities for terminating the growth of these
bacterias. The anti-bacterial activity of synthesized ZnO-NBs was
tested against Gram +ve bacteria, B. subtilis (MTCC 121). Though
B. subtilis is not as much harmful as other pathogenic bacterias,
but it is an easy model for doing the clinical studies and checking
the anti-bacterial activity as the mechanism for attacking on the Gram
+ve bacterial colonies are generally same 31-42 .
5.2.2 Anti-bacterial activity of ZnO-NBs
The anti-microbial activities were studied by Disc diffusion
method with the help of nutrient agar along with suspended
microbes, by dipping the pad into the culture. After streaking the
agar plate, these are incubated for 24 hrs. Antibiotic discs were kept on
agar surfaces and the anti-bacterial activities were noted from the
inhibition zones (IZ) results and finally the calculation of diameters of
IZ of stains were carried out. The ZnO Nano-bunches (ZnO-NBs)
fabricated at different temperature ranges were taken and their
colloidal suspensions were tested for their activity against Bacillus
subtilis. For this purpose the filter paper disc technique was exploited.
Prior to this, the ZnO-NBs were attempted with bactericidal activity by
disc diffusion method for Bacillus subtilis. The cultures used were
kept over night at 37 °C on nutrient agar. On preceding the process,
the cultures were centrifuged at moderate rpm range and the pellets
were diluted in NSS to get the count more than 100 cfu/ ml. The
bacterial culture suspension was extended on the plates of Nutrient
agar homogeneously and 8 mm discs of Hi-media Pvt Ltd make were
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Figure 5.1 Antimicrobial activities of ZnO-NBs prepared at different temperature ranges against Gram-positive (Bacillus subtilis)
made sterile for ZnO-NBs. These plates were further kept at 37 °C
for another day. Standard anti-biotic discs of 0.03 mg each and
doxycycline were utilized and ZnO-NBs media of similar
concentrations were set as control. Later the resulting inhibition
zones (IZ), in mm of bacterial growth were calculated for the
determination of anti-bacterial activities.
The influence of metal oxide nano-particulates (MO-Nps) with
the pathogenic bacterias is an evolving area for research.
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Consequently, after fruitful characterization results, the peanut shaped
ZnO nano-bunches fabricated at different temperature range were
tested for their bactericidal performances. The different proportions
of ZnO nano-bunches exhibited significantly anti-bacterial activity
against the Gram +ve bacteria, Bacillus subtilis, responsible for
producing the heat stable toxin amylopsin related to food borne illness.
The concentration dependent anti-bacterial activity shown in terms of
zone of inhibition was visible on NA plates (Figure 5.1). At highest
concentration, the growth of bacterial strains, Bacillus subtilis was
stop down appreciably (Table 5.1).
Table 5.1 Anti-bacteril activity of Peanut-shaped ZnO-NBs prepared at different temperature ranges; TEMP-I and TEMP-II, against B. subtilis
(MTCC 121)
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At 500 g/ml of Peanut shaped ZnO nano-bunches, maximum zone of
inhibition (IZ) i.e. 22 mm was recorded for B. subtilis (MTCC 121)
by ZnO-NBs fabricated at TEMP-II. After that the maximum zone of
inhibition (IZ) of about 20 mm was recorded for B. subtilis
(MTCC 121) by Peanut-shaped ZnO nano-bunches fabricated at
TEMP-I. The initial readings also postulate that the anti-bacterial
performances of ZnO-NBs may be dependent of size & morphology of
peanut-shaped ZnO-NBs.
The bactericidal properties of these peanut shaped ZnO-NBs
recommend that these nano-bunches were more diffusible in the
growth culture medium which allows the superior interferences among
bacterial cells and that of nano-bunches. Similar anti-bactericidal
impacts of some other ZnO nano-particles fabricated from other
techniques are also known 27-43. The stains of Gram+ bacteria, Bacillus
subtilis, were most vulnerable to the ZnO nano-particulates. This
might be due to presences of some proteins and polysaccharides on
surface of cell wall of the bacteria. Therefore the Peanut-shaped
ZnO-NBs action on B. subtilis changes the cell wall morphology as
well as its permeability, ultimately leading to the bacterial growth by
its death.
5.2.3 Conclusion
In addition to all, the synthesized peanut-shaped ZnO-NBs
also showed significant antimicrobial activity suggesting that peanut-
shaped ZnO-NBs can be better agents to control the spreading of
bacterial infections. Though B. subtilis is not as much harmful as
other pathogenic bacterias, but it is an easy model for doing the clinical
studies and checking the anti-bacterial activity, as the mechanism
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for attacking on the Gram +ve bacterial colonies are same. Therefore,
we can postulate that our synthesized peanut-shaped ZnO-NBs may be
externally used in a variety of therapeutic formulations as it is treated
for potential interventions in extrinsic skin problems like
aging,sunburn, etc.
5.3 Photo-catalytic Activity of ZnO Nanostructures (Ns)
5.3.1 Introduction
With the emergence of industrialization, technological
advancements and different techniques used for agricultural
productivity may directly or indirectly leading to cause harmful results
in the environment. The different types of dangerous compounds
eluting from the drugs, dyes, surfactants (used in shampoos, rinses,
etc.), pharmaceutical and many other chemical/gas industries;
insecticide, fungicides, herbicides, etc from agri-practices; all may
resulting into the huge contamination of water bodies, soil, air, etc.
Hence, for the sake of mankind, flora and fauna, it is necessary that all
the harmful and poisonous contaminants going into different water
bodies (viz. surface water, underground water, etc), could be
checked and sound initiatives should be taken to purify the
different water resources, used for the betterment of life. Photo-
catalysis means the use of photon energy in the catalytic reactions
(like photosynthesis in plants). Since few years, photo-catalytic
processes involving semiconductor ZnO Nanostructures under UV
light illumination have been shown to be potentially beneficial and
helpful in the treatment of various hazardous pollutants. Different
studies have proved this with different pollutants like dyes, drugs,
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surfactants, pesticides, herbicides, insecticides and fungicides that can
be completely mineralized in the presence of ZnO Nanostructures
44-53.
5.3.2 Mechanism
As far as the mechanism of photo-catalytic degradation is
concerned, it is comparable with the mechanism of chlorophyll in
photosynthesis. On the exposure of light/photon energy (where hߥ
> 3.2 eV) on the ZnO Nanostructures; free electrons (e--) find its
position at Conduction state, leaving behind holes (h+) at the
Valence state. These further leads to the oxidation-reduction
reactions between the pollutants and ZnO Nanostructures, both present
in the water solution and hence the photo-catalysis of pollutants
(viz. drug, dye, surfactatnt, etc) occurs.
ZnO(Ns) + hv e + h+ Eq. (5.1)
O2 + e O2 Eq. (5.2)
H2O + h+ OH + H+ Eq. (5.3)
It is the oxidation reactions which are responsible for the photo-
catalytic degradation of harmful drugs, dyes, etc. Both the free
electrons (e--) as well as the free holes (h+), interact with Oxygen
and Water molecules, to form radicals of superoxides’ and
hydroxyl groups respectively. A good amount of O2 vacancies can
catch semi-conductor electrons leading to oxidation of organic
substances. On the other hand the hydroxyl groups increases with
the decrease in semi-conductor electron densities. This overall
increases the photo-catalytic activity of ZnO Nanostructures.
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Previous studies support the degradation of diverse pollutants by
ZnO Nanostructures efficiently 43-73. Y Lv et al. synthesized ZnO
Nanostructures and studied its photo-catalytic activity in degradation
of rhodamine B (RhB) and expedient recycling of the synthesized
ZnO-Ns provide it potential in waste water treatment. R. Saravanan et
al. showed the photo-catalytic activity of the synthesized ZnO
Nanorods for the degradation of methylene blue and methyl orange
72. R. Ullah and J. Dutta reported that Mn-doped ZnO (ZnO:Mn2+)
was more effective for the photo-catalytic degradation of Methylene
Blue than the undoped ZnO upon its visible light exposure 62. C.
Hariharan et al. showed that the listed aromatic & aliphatic chloro-
compounds (listed water contaminants) can reduce the used ZnO
Nanstructures, they reported an effective procedure to identify
and degrade these contaminants 63. Our study is also based on the
mechanism exploited by the scientists earlier, using ZnO
Nanostructures for photo-catalytic activity.
5.3.3 Photo-catalytic Degradation of Acid Red 183 in the
presence of ZnO Nano-whiskers (ZNWs)
Under UV light source, the photocatalytic activity of the ZnO
Nano-whiskers (ZNWs) was investigated by studying the
degradation of Acid Red 183 as a function of irradiation time.
5.3.3.1 Procedure
Stock solution of the dye derivative containing desired
concentration was prepared in double distilled water. An
immersion well photochemical reactor made of Pyrex
glass equipped with a magnetic stirring bar, water circulating
jacket and an opening for supply of atmospheric oxygen was
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used. The aqueous solution of the dye (250 mL) with 0.3 mM
concentration was taken into the photoreactor and required
amount (2 gL-1) of photocatalyst was added for
irradiation experiment. The solution was stirred bubbled
with atmospheric oxygen for at least 15 minutes in the dark
to allow equilibration of the system so that the loss of
compound due to adsorption can be taken into account. The
zero time reading was obtained from blank solution kept in
the dark but otherwise treated similarly to the irradiated
solution. The suspension was continuously purged with
atmospheric oxygen throughout each experiment.
Irradiation was carried out using a 125 W medium pressure
mercury lamp. The light intensity falling on the solution
was measured using a UV-light intensity detector (Lutron
UV-340) and was found to be 1.74-1.78 m Wcm-2. IR
radiation and short wavelength UV radiation were
eliminated by a water circulating Pyrex glass jacket.
Samples (10 mL) were collected before and at
regular intervals during the irradiation and analyzed after
centrifugation.
5.3.3.2 Characterization
The photo-decolourization of acid red 183 was
monitored by measuring the absorbance at their max as a
function of irradiation time using UV spectroscopic
analysis technique (Shimadzu UV-Vis 1601).
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5.3.3.3 Experimental
The different types of experimental conditions have been
placed to draw three sets of results. The three types of experimental
condition along with their graph are as:
5.3.3.3.1 Experimental conditions – 1
Reaction vessel : immersion well
photochemical reactor made of Pyrex glass, light source:
125 W medium pressure mercury lamp (1.74- 1.78 mWcm-2),
absorbance was followed at 494 nm, photocatalyst : ZnO Nano-
whiskers (ZNWs), Acid red 183 (0.30 mM), volume (250
mL), continuous stirring and air purging, irradiation time: 150 min
5.3.3.3.2 Experimental condition - 2
Reaction vessel : immersion well
photochemical reactor made of Pyrex glass, light source: 125
W medium pressure mercury lamp (1.74-1.78 mWcm-2),
absorbance was followed at 494 nm, photocatalyst: ZnO Nano-
whiskers (ZNWs), Acid red 183 (0.30 mM), volume (250
mL), continuous stirring and air purging, irradiation time: 150 min.
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Figure 5.2 Change in absorbance on irradiation of an aqueous suspension of
acid red 183 in the presence of ZnO Nano-whiskers (ZNWs)
5.3.3.3.3 Experimental condition – 3
Reaction vessel : immersion well
photochemical reactor made of Pyrex glass, Acid red 183
(0.30 mM), volume (250 mL), photocatalyst: ZnO Nano-whiskers
(ZNWs) (1 gL-1), Sachtleben Hombikat UV100 (1 gL-1),
Millennium PC500 (1 gL-1), light source: 125 W medium pressure
mercury lamp (1.74-1.78 mWcm-2), continuous stirring and air
purging, irradiation time: 150 min.
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Figure 5.3 Change in concentration as a function of time on irradiation of an
aqueous solution of acid red 183 in the presence and absence of ZnO Nano-
whiskers (ZNWs)
5.3.3.2 Result and Discussions
An aqueous solution of dye derivative, acid red 183 (0.3 mM,
250 mL) on irradiation with a 125 W medium pressure
mercury lamp in the presence of ZnO Nano-whiskers (ZNWs)
(2 gL-1) with constant stirring and bubbling of atmospheric
oxygen lead to the decolourization of the dye. Experimental
condition – 1 (Figure 5.2) shows the change in absorbance at
different time interval on irradiation of Acid red 183 in the
presence of ZNWs where the absorption intensity
decreases with increasing irradiation time.
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Figure 5.5 Comparison of decolourization rate of acid red 183 in the presence of different types of photo-catalysts
Experimental condition – 2 (Figure 5.3) shows the change in
concentration of Acid Red 183 in the presence and
absence of ZnO as a function of irradiation time. [The
concentration of dye was calculated by standard calibration curve
obtained from the absorbance of the dye at different
known concentration]. It could be seen from the figure that 68%
decolourization takes place after 150 min of irradiation in
the presence of ZnO Nano-whiskers. Blank experiment
was carried out by irradiating the aqueous solution
of the compound in the absence of photocatalyst, where no
observable decrease in the dye concentration could be seen.
Experimental condition – 3 (Figure 5.4) shows the comparison of
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decolourization rate of Acid red 183 in the presence of different
types of photo-catalysts viz. ZnO Nano-whiskers (ZNWs), UV100,
PC500. It was found that the synthesized ZNWs shows the maximum
decolourization rate of acid red 183.
5.3.3.3 Conclusion
The photo-degradation of Acid Red 183 by using the photo-
catalyst, ZnO Nano-whiskers (ZNWs) showed good results of
absorption and 68% of decolouration takes place after 150 min of
irradiation in the presence of ZnO Nano-whiskers experiments. Finally
it was also observed that the decolourization rate of Acid red 183
by ZNWs was maximum, when compared with other photo-catalysts.
The versatile nature of ZnO Nano-whiskers (ZNWs) leads to its
explorations in different applications. Whether it may be anti-microbial
activity shown by nano-scaled ZnO or it may be the functions of sensing
various biological molecules, chemicals, gaseous, etc; whether it may be
photo-catalytic activity by ZnO against different pollutants or it may be anti-
cancerous activity or the functioning as solar cells; so on and so forth. Its
huge applications perspective proved its vast requirement worldwide.
Furthermore, enhancement of its quality characteristic with improvement in
its properties further leads to its better usage. Since few decades, with the
advent of nanotechnology, the applications of nano-scaled ZnO have been
mounting rapidly. But still there is a big room left for various scientific
groups worldwide, in order to present more efficient and innovative
technologies based on ZnO Nanostructures.
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