International Journal of Inorganic Materials 2 (2000) 451–454

Change in antibacterial characteristics with doping amount of ZnO in

MgO–ZnO solid solution

*

Osamu Yamamoto , Jun Sawai, Tadashi Sasamoto

Department of Applied Chemistry

, Kanagawa Institute of Technology, 1030 Shimo-ogino, Atsugi-shi, 243-0292 Japan

Accepted 11 May 2000

Abstract

Antibacterial activity for MgO–ZnO solid solution was studied by measuring the change in electrical conductivity with bacterial

growth. MgO–ZnO solid solution powders were prepared by heating at 14008C for 3 h in air. A single phase with cubic type structure was
obtained in the weight ratio range (MgO / ZnO) of 4.0 and 1.5, but the ratio of 0.67 resulted in a ZnO phase in addition to solid solution.
After milling the solid solution powders by planetary ball mill, the average particle size and the specific surface area of these powders

2

became 0.1 mm and 26 m / g, respectively, which were used in the test of antibacterial activity. From the results of antibacterial tests, the
activity increased with increasing the powder concentration in the medium. With increasing the doping amount of ZnO in MgO–ZnO
solid solution, it was found to show a decrease in the antibacterial activity against Escherichia coli and Staphylococcus aureus. The pH
value in physiological saline at the powder concentration of 2.5 mg / ml showed the alkali region above 10.0, and decreased with the
increase of ZnO amount in solid solution. The decrease in antibacterial activity, therefore, was associated with the decrease of pH value in
medium

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2000 Elsevier Science Ltd. All rights reserved.

Keywords

: MgO; ZnO; Solid solution; Antibacterial activity

1. Introduction

cancellation in activity, because these three ceramics are
known to form solid solutions easily.

Microbial pollution and contamination that take place by

In the present work, MgO–ZnO solid solution powders

microorganisms have produced various problems in indus-

were prepared with the various weight ratios (MgO / ZnO)

try and other vital fields, including medicine, such as

at 14008C for 3 h in air. After preparing the slurries of the

degradation and infection, etc. In order to solve these

powders obtained, the change in antibacterial activity as a

problems, therefore, new pasteurization and antibacterial

function of MgO–ZnO composition in the solid solution

techniques have been demanded and studied [1–3].

was studied by measuring the change in electrical con-

Recently, the occurrence of antibacterial activity by

ductivity with bacterial growth.

using ceramic powders has been pointed out with much
attention as a new technique that can substitute for
conventional ones using organic agents. Ceramic powders

2. Experimental

of zinc oxide (ZnO), calcium oxide (CaO) and magnesium
oxide (MgO) were found to show a marked antibacterial

2.1. Preparation of samples and test bacteria

activity without the presence of light [4–7]. The use of
these ceramics has the following advantages; mineral

MgO and ZnO powders of reagent grade were used as

elements essential to the human body and strong anti-

starting materials. These powders were mixed with differ-

bacterial activity in small amount without the irradiation of

ent weight ratio (MgO / ZnO) and then heated at 14008C

light [8–10]. However, it is not yet clear what change in

for 3 h in air. The as-prepared powder samples of MgO–

antibacterial activity is expected by the formation of solid

ZnO solid solution were milled by planetary ball mill. The

solution among these powders, either reinforcement or

sample code and the chemical composition of solid
solution are listed in Table 1. In order to examine the
formation of solid solution, X-ray diffraction measurement

*Corresponding author. Tel.: [phone]; fax: [phone]-

8760.

(XRD) was carried out and the specific surface area of

1466-6049 / 00 / $ – see front matter

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2000 Elsevier Science Ltd. All rights reserved.

P I I : S 1 4 6 6 - 6 0 4 9 ( 0 0 ) 0 0 0 4 5 - 3

452

O

. Yamamoto et al. / International Journal of Inorganic Materials 2 (2000) 451 –454

Table 1

measuring the change in electrical conductivity with

Sample code and chemical composition of solid solution powders used in

bacterial growth. The apparatus for measuring the con-

this study

ductivity was Bactometer Microbial Monitoring System

Sample code

Mass ratio

Model 64 (company: bioMerieux) as shown in Fig. 1.

MgO:ZnO

Preparation of bacteria into the wells of a module for the

SS-10

1:0

Bactometer was carried out as follows; adding the powder

SS-82

8:2

samples into the well containing Modified Plate Count

SS-64

6:4

Agar (MPCA) and then dispensing the bacterium suspen-

SS-46

4:6

sion into the well. After setting the module in the
Bactometer, the change in electrical conductivity was

powder samples was measured. The powder samples

monitored during the incubation at 378C for 25 h in a dark

obtained were suspended with physiological saline in the

place. The details of the procedures were reported in a

concentration range from 1.6 to 100 mg / ml and then the

previous paper [4].

slurries prepared were used in antibacterial tests.

In order to examine indirectly the pH values when the

Staphylococcus aureus 9779 (S

. aureus) and Escherichia

powder samples were added into the well, the samples

coli 745 (E

. coli) were used as test bacteria and stored at

were dispersed into physiological saline at a powder

Tokyo Metropolitan Research Laboratory of Public Health.

concentration of 2.5 mg / ml. After keeping the dispersed

These bacteria were cultured in Brain Heat Infusion (BHI)

solutions for 24 h, the pH values of physiological saline

at 37C for 24 h on a reciprocal shaker. The bacterial

were measured.

culture was suspended in a sterile physiological saline with

3

a final concentration of approximately 10 CFU / ml (CFU:
Colony Forming Unit).

3. Results

2.2. Test of antibacterial activity

3.1. Powder samples

Antibacterial activity of powder samples was judged by

In Fig. 2, XRD patterns of the powder samples are

shown. A single phase of solid solution with NaCl type
structure was formed in SS-82 and SS-64 samples. For the
SS-46 sample, however, ZnO with the hexagonal wurtzite

Fig. 2. XRD patterns of MgO–ZnO solid solution heated at 14008C for 3

Fig. 1. Schematic illustration of the apparatus used in antibacterial test.

h in air.

O

. Yamamoto et al. / International Journal of Inorganic Materials 2 (2000) 451 –454

453

7

type structure coexisted with the solid solution of the cubic

change occurs at the bacterial concentration of about 10

phase with the NaCl type structure. This result agreed with

CFU / ml in the medium.

the phase diagram [11]. A diffraction line of solid solution

Fig. 3(a) and (b) show the changes in electrical con-

with the index of 200 shifted to high-angle side with

ductivity of SS-10 and SS-64, respectively, with the

increasing the amount of ZnO and the lattice constant of

incubation time of S

. aureus. In the figures, DT (Detection

the cubic phase changed from 0.211 to 0.204 nm. This

Time) shows the incubation time at which an electrical

21

shift is considered to be due to the replacement of Mg

change can be detected. Hence, if the value of DT is

21

ions (ion radius: 0.065 nm) with larger Zn

ions (ion

delayed by adding the powder samples, it can be judged

radius: 0.074 m). ZnO detected in SS-46 sample seems to

that the samples have the effect of an inhibition of the

be due to excess ZnO in the formation of solid solution.

After milling the powder samples by planetary ball mill,

it was found that the specific surface area and particle size

2

was about 26 m / g and 0.1 mm, respectively.

3.2. Antibacterial activity of powder samples

With the growth of bacteria, it is known that electrolytes

such as organic and amino acids are produced with the
digestion of proteins in the medium [12]. The electrical
conductivity in such a growth medium, therefore, increases
with an increase of the electrolytes produced, of which the

Fig. 3. The change in electrical conductivity with incubation time on S

.

Fig. 4. Comparison of antibacterial activity against (a) S

. aureus and (b)

aureus. (a) SS-10 and (b) SS-64 sample.

E

. coli by adding powder samples.

454

O

. Yamamoto et al. / International Journal of Inorganic Materials 2 (2000) 451 –454

bacterial growth. In the case of no addition of SS-10

ZnO in MgO–ZnO solid solution. From this result, it is

(control), the DT value was approximately 15 h. By adding

found that bacteria growth is inhibited with increasing the

SS-10, however, the DT value increased with the increase

pH value in the medium.

of powder concentration and no DT value was detected in

Sawai et al. [16] have even found the generation of

2

the powder concentration of 6.3 mg / ml (see Fig. 3(a)). The

super-oxide, O , from the surface of MgO and considered

2

change of DT value of SS-64 was similar to that of SS-10

to have an effective activity for the inhibition of bacterial

and no DT was observed in powder concentration of 12.5

growth. And also, it has been known that the super-oxide

mg / ml (see Fig. 3(b)). The results indicate an increase in

is stable under alkali region and then the diffuse distance

the antibacterial activity against S

. aureus by increasing

of super-oxide increases with increasing pH value [17]. In

the powder concentration in medium.

the present work, therefore, super-oxide is expected to

Based on the change in electrical conductivity described

generate from the surface of MgO–ZnO solid solution.

above, the antibacterial activity of all powder samples

Based on the above discussion, the decrease in the

prepared was examined on two bacteria, S

. aureus and E.

antibacterial activity with increasing ZnO content in solid

coli.

solution is anticipated to be due to the decrease of stability

2

Fig. 4(a) and (b) show the comparison of antibacterial

of O

generated from the surface of the solid-solution and

2

activity of four samples on S

. aureus and E. coli, respec-

the decrease of pH value in the medium.

tively. The vertical axis, ‘DT / DT

’ represents the ratio

In conclusion, by measuring the change in electrical

control

of the DT value at specified concentration of powder

conductivity with bacterial growth, it was found that the

samples to that at no addition of powder samples (control).

antibacterial activity against Escherichia coli and Staphy-

If the values of DT / DT

are changed with a steep rise

lococcus aureus decreased by increasing the doping

control

at the lower powder concentration, it can be judged to

amount of ZnO in MgO–ZnO solid solution.

show the stronger antibacterial activity. As shown in Fig.
4(a), with the increase of ZnO amount in solid solution, the
pronounced change of the value is observed at high

Acknowledgements

powder concentration, that is, the decrease of ZnO in solid
solution results in an effective antibacterial activity on S

.

The authors thank Professor Michio Inagaki of Aichi

aureus. In E

. coli (see Fig. 4(b)), the change of DT /

Institute of Technology for his discussion and encourage-

DT

value occurs at a little higher powder concen-

control

ment.

tration with the increase of ZnO amount in solid solution
than in S

. aureus. The change of antibacterial activity of

powder samples on E

. coli was similar to those on S.

aureus.

References

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4. Discussion

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