Materials Chemistry and Physics 121 (2010) 198–201

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Materials Chemistry and Physics

j o u r n a l h o m e p a g e :

w w w . e l s e v i e r . c o m / l o c a t e / m a t c h e m p h y s

ZnO nanofluids: Green synthesis, characterization, and antibacterial activity

Razieh Jalal

a

, Elaheh K. Goharshadi

a

,

b

,

∗

, Maryam Abareshi

a

, Majid Moosavi

c

,

Abbas Yousefi

d

, Paul Nancarrow

e

a

Dept. of Chemistry, Ferdowsi University of Mashhad, Mashhad 91779 Mashhad, Iran

b

Center of Nano Research, Ferdowsi University of Mashhad, Iran

c

Dept. of Chemistry, Faculty of Sciences, University of Isfahan, Isfahan 81746-73441, Iran

d

Pare-Tavous Research Institute, Mashhad, Iran

e

QUILL Research Centre and School of Chemistry and Chemical Engineering, Queen’s University Belfast, UK

a r t i c l e i n f o

Article history:
Received 9 April 2009
Received in revised form
20 December 2009
Accepted 8 January 2010

Keywords:
Nanostructures
Chemical synthesis
Ionic liquid
Antibacterial activity

a b s t r a c t

Zinc oxide nanoparticles have been synthesized by microwave decomposition of zinc acetate precursor
using an ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [bmim][NTf

2

] as

a green solvent. The structure and morphology of ZnO nanoparticles have been characterized using X-ray
diffraction and transmission electron microscopy. The ZnO nanofluids have been prepared by dispersing
ZnO nanoparticles in glycerol as a base fluid in the presence of ammonium citrate as a dispersant. The
antibacterial activity of suspensions of ZnO nanofluids against (E. coli) has been evaluated by estimating
the reduction ratio of the bacteria treated with ZnO. Survival ratio of bacteria decreases with increasing
the concentrations of ZnO nanofluids and time. The results show that an increase in the concentrations
of ZnO nanofluids produces strong antibacterial activity toward E. coli.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Room-temperature ionic liquids (RTILs) are very promising

replacements for the traditional volatile organic solvents due to
their high mobility, low melting points, negligible vapor pressure,
thermal stability, low toxicity, large electrochemical window, non-
flammability, and ability to dissolve a variety of chemicals

[1–4]

.

Their quite rapid emergence as alternative solvents has involved
a rapidly growing number of examples of application in organic
chemistry but the use of RTILs in inorganic synthesis is still in its
infancy. Only in recent years, the advantages of RTILs in inorganic
synthetic procedures have gradually been realized and received
more and more attention. Various nanostructured materials, such
as palladium

[5]

, iridium

[6]

, gold

[7]

, tellurium

[8]

, TiO

2

[9,10]

, ZnO

[11–14]

, and CoPt

[15]

have been synthesized in RTILs.

Due to the outbreak of the infectious diseases caused by dif-

ferent pathogenic bacteria, the scientists are searching for new
antibacterial agents. In the present scenario, nanoscale materials
have emerged up as novel antimicrobial agents owing to their high
surface area to volume ratio and the unique chemical and physical
properties

[16]

.

∗ Corresponding author at: Department of Chemistry, Ferdowsi University of

Mashhad, Mashhad 91779, Khorasan Razavi, Iran. Tel.: [phone];
fax: [phone].

E-mail address:

[e-mail]

(E.K. Goharshadi).

In recent years, the use of inorganic antimicrobial agents has

been attracted interest for the control of microbes. The key advan-
tages of inorganic antimicrobial agents are improved safety and
stability, as compared with organic antimicrobial agents

[17]

.

At present, most antibacterial inorganic materials are metallic
nanoparticles

[18–20]

and metal oxide nanoparticles such as zinc

oxide

[17]

.

In this work, a green and cost-effective method for the prepara-

tion of ZnO nanoparticles via microwave-assisted decomposition
of zinc acetate precursor using 1-butyl-3-methylimidazolium
bis(trifluoromethylsulfonyl) imide, [bmim][NTf

2

] as a solvent has

been used. The morphology and structure of ZnO nanoparticles
have been characterized using transmission electron microscopy
(TEM) and X-ray diffraction (XRD), respectively. ZnO nanofluids
have been prepared by dispersing ZnO nanoparticles in glycerol
as a base fluid in the presence of ammonium citrate as a dispersant.

The antibacterial activity of ZnO nanofluids was tested against

Escherichia coli at different concentrations by colony count method.
The quantitative examination of bacterial activity has been esti-
mated by the survival ratio (N/N

0

) as calculated from the number

of viable bacterial cells (N) at specified time and the initial viable
cells (N

0

), which form colonies on the nutrient agar plates.

2. Experimental

2.1. Materials

In our experiments, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)

imide was used as a solvent, which was synthesized according to the literature

[21]

.

0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:

10.1016/j.matchemphys.2010.01.020

R. Jalal et al. / Materials Chemistry and Physics 121 (2010) 198–201

199

All other chemicals used were of analytical grade and purchased and used as received
without further purification.

2.2. Preparation of ZnO nanoparticles

Zinc acetate dihydrate (5.5 g) was dissolved in 50 mL of deionized water, and

then solid NaOH (16 g) was added slowly into the zinc acetate dehydrate solution
under magnetic stirring at room temperature and formed a transparent Zn(OH)

4

2

−

solution. Then 2 mL of [bmim][NTf

2

] was added to 3 mL of the above solution. The

suspension was put into a domestic microwave oven (2.45 GHz, 850 W) in air, 30% of
the output power of the microwave was used to irradiate the mixture for 5 min (on
for 10 s, off for 5 s). The white precipitate was collected by centrifugation, washed
with deionized water and ethanol several times, and dried in vacuum oven at 40

◦

C

for 10 h.

2.3. Preparation of suspension of ZnO nanoparticles

Suspensions of ZnO nanoparticles were prepared with glycerol with the aid of

a magnetic stirrer. To enhance the stability of the suspensions, ammonium cit-
rate was used as a dispersant. In every sample, the weight ratio of dispersant to
nanoparticle was kept 1:1. The samples were stable at least for several months and
no agglomeration and sedimentation of the particles in the samples was observed.

2.4. Testing of antibacterial activity

E. coli DH5

␣ was grown aerobically at 37

◦

C for overnight with shaking in an ordi-

nary broth medium containing 0.5% yeast extract, 1% bactopeptone, and 1% sodium
chloride. The saturated culture was first diluted in fresh LB medium and incubated
at 37

◦

C for 3–4 h on a reciprocal shaker. Subsequently, the solution of bacterial sus-

pension was added into LB medium with a final concentration of 10

7

CFU dm

−3

(CFU:

colony forming unit) containing ZnO nanoparticles with concentration in the range
from 0.125 to 0.5 g dm

−3

and then was kept at 37

◦

C for different times on a recip-

rocal shaker. After sampling the bacterial suspension of 1

× 10

−4

dm

3

, the bacterial

suspension was spread on nutrient agar, and cultured at 37

◦

C for 48 h without the

presence of light. The colony formed with bacterial growth was counted. By calculat-
ing the ratio (N/N

0

) between the viable bacterial counts (N (CFU dm

−3

)) at specified

time and the initial counts (N

0

(CFU dm

−3

)) of bacteria, antibacterial activity was

evaluated. All results were compared with two controls: (1) a fluid containing glyc-
erol and ammonium citrate without ZnO nanoparticles and (2) a fluid without ZnO
nanoparticles in the absence of base fluid and dispersant.

3. Results and discussion

3.1. Characterization of the ZnO nanoparticles

The structural properties of the products were analyzed using

D8 Advanced diffractometer with Cu K

␣ radiation ( = 0.15406 nm).

Fig. 1

shows the XRD pattern of nanoparticles. All the peaks of the

(1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1),
(0 0 4), and (2 0 2) reflections can be indexed to the known hexago-
nal wurtzite structure of ZnO with lattice constants of a = b = 3.242 Å
and c = 5.205 Å. These match well with those in the JCPDS card (Joint
Committee on Powder Diffraction Standards, Card No. 89-1397).
The strong intensity and narrow width of ZnO diffraction peaks
indicate that the resulting products were of high crystalline.

Fig. 1. Powder X-ray diffraction pattern for the ZnO nanoparticles.

Although, weak dispersion peaks of background can be observed

due to some amorphous material in the sample, no diffraction peaks
from other species could be detected in the XRD pattern, which
indicates that all the precursors have been completely decomposed
during the decomposition process.

The size of nanoparticles can be estimated from the Scherrer

equation:

D

h k l

=

k × 

ˇ

h k l

× cos 

h k l

(1)

where D

h k l

is the particle size perpendicular to the normal line of

(h k l) plane, k is a constant (it is approximately equal to 0.9),

ˇ

h k l

is the full width at half maximum (FWHM) of the (h k l) diffraction
peak,



h k l

is the Bragg angle of (h k l) peak and

 is the wavelength

of X-ray. The mean size of ZnO nanoparticles was calculated using
Eq.

(1)

. The peak position and the FWHM were obtained by fit-

ting the measured peaks with two Gaussian curves in order to find
the true peak position and width corresponding to monochromatic
Cu K

␣ radiation. The mean particle sizes for the crystallographic

planes (1 0 1), (0 0 2), and (1 0 0) were 41.30, 47.52, and 37.15 nm,
respectively.

The transmission electron microscopy (TEM) was recorded by

the LEO system (model 912 AB) operating at 120 kV for samples.

Fig. 2

is a TEM image of the ZnO nanoparticles. It can be seen that

the uniform nanocrystalline ZnO particles have sphere shapes with
weak agglomeration.

3.2. Biological activity

Autoclaved ZnO nanoparticles were mixed with autoclaved LB

medium to make nanofluids with ZnO concentrations of 0.125, 0.25,
and 0.5 g dm

−3

.

Fig. 3

shows the image of bacteriological tests of

E. coli on solid agar plates without ZnO nanoparticles and with
0.5 g dm

−3

ZnO suspensions. The results show that a number of

E. coli colonies appeared on the solid agar plates without ZnO
nanoparticles; however, several E. coli colonies are observed on the
solid agar plates with 0.5 g dm

−3

revealing that ZnO nanoparticles

in glycerol solution can restrain E. coli proliferation. The survival
ratio decreases with increasing ZnO nanoparticles concentrations

Fig. 2. TEM image of ZnO nanoparticles.

200

R. Jalal et al. / Materials Chemistry and Physics 121 (2010) 198–201

Fig. 3. Images of bacteriological tests of E. coli on solid agar plates with 0.5 g dm

−3

ZnO suspensions (left) and without ZnO nanoparticles (right) in the presence of glycerol

and ammonium citrate.

(see

Fig. 4

(a)), indicating that antibacterial activity increases with

increasing nanoparticles concentration. When the concentration
of nanofluid was 0.125 g dm

−3

, the ratio did not decrease even

after 120 min. The survival ratio when the concentration was 0.25
or 0.5 g dm

−3

decreased after 20 min and reached to the mini-

mal values after 40 (0.72) and 120 min (0.21), respectively. The
ratio in 0.5 g dm

−3

concentration was steeper decrease than that

of 0.25 g dm

−3

. These results indicate that an increase in nanopar-

ticles concentration produces strong antibacterial activity toward
E. coli.

On occurrence of antibacterial activity on ZnO, Sawai et al.

[22]

,

Yamamoto

[23]

, Ohira et al.

[24]

, and Padmavathy and Vijayaragha-

van

[25]

reported that the generation of H

2

O

2

from the surface of

ZnO was one of the primary chemical species being responsible for
antibacterial action.

The generation of highly reactive species such as OH

−

, H

2

O

2

,

and O

2

2

−

is explained as follows

[25]

. Since ZnO with defects can

be activated by both UV and visible light, electron–hole pairs (e

−

h

+

)

can be created. The holes split H

2

O molecules from ZnO nanofluid

into OH

−

and H

+

. Dissolved oxygen molecules are transformed to

superoxide radical anions (

•

O

−2

), which in turn react with H

+

to

generate (HO

2

•

) radicals, which upon subsequent collision with

electrons produce hydrogen peroxide anions (HO

2

−

). Then they

react with hydrogen ions to produce molecules of H

2

O

2

. The gen-

erated H

2

O

2

can penetrate the cell membrane and kill the bacteria

[25]

. Since, the hydroxyl radicals and superoxides are negatively

charged particles, they cannot penetrate into the cell membrane
and must remain in direct contact with the outer surface of the bac-
teria; however, H

2

O

2

can penetrate into the cell

[25]

. It is plausible

to say that the concentration of nanofluids is comparable with the
amount of H

2

O

2

. The amount of H

2

O

2

generated from the surface of

ZnO should increase in proportion to increase of the nanoparticles
concentration and time as mentioned by the previous report

[23]

.

The reason for increasing the antibacterial activity with increas-
ing concentration of ZnO nanofluid and time is assumed due to the
increase of H

2

O

2

concentration generated from the surface of ZnO.

Fig. 4

(b) shows the changes in the survival ratio in the case of

two controls, without ZnO nanoparticles in the presence or absence
of glycerol and ammonium citrate. There were no statistically
significant differences between the absence, 0.25 and 0.5 g dm

−3

of ammonium citrate although 0.5 g dm

−3

was more effective in

growth reduction of E. coli than that of 0.25 g m

−3

. The ammonium

citrate at a concentration of 0.5 g dm

−3

caused a reduction in the

Fig. 4. Changes in survival ratio of E. coli by using a concentration of 0.25 and
0.5 g dm

−3

(a) without ZnO nanoparticles (b) without ZnO nanoparticles in the

absence of (circle symbol) and in the presence of solvent, glycerol and ammonium
citrate (the amount of solvent for triangle and square symbols are the same as those
of (a)).

R. Jalal et al. / Materials Chemistry and Physics 121 (2010) 198–201

201

number of bacterial colonies and could inhibit bacterial growth by
about 19.7% after 120 min compared with the absence of ammo-
nium citrate. The antibacterial activity of citrate salts against E. coli
has been reported

[26,27]

.

4. Conclusions

ZnO nanofluids have been prepared in glycerol as a base fluid

in the presence of ammonium citrate as a dispersant. By careful
examination of the results, it seems to us that this work contains
the following conclusions:

1. Microwave-assisted heating method has been successfully

established for the preparation of nanocrystalline ZnO in the
presence of an ionic liquid. The method is found to be convenient,
mild, efficient, and environmentally friendly.

2. Our experimental results revealed that low concentra-

tions of ZnO nanofluids could not inhibit bacterial growth.
The antibacterial activity of ZnO increases with increasing
nanofluid concentration and time. In the antibacterial tests,
it was found that the ZnO nanofluids are good bactericidal
agents.

3. Ammonium citrate could slightly, but not significantly, reduce

the number of colonies on the agar plates. Hence, ammonium
citrate can act as a weak antibacterial agent.

Acknowledgements

The authors gratefully acknowledge the support of the Research

Committee of Ferdowsi University of Mashhad. The authors
would also like to thank Mrs. Roksana Pesian for taking TEM
image.

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