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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

169

[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.

172

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

174

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.

176

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

177

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).

178

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.

179

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.

180

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

182

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.

183

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