Journal of Biomaterials and Nanobiotechnology, 2013, 4, 365-373
http://dx.doi.org/10.4236/jbnb.2013.44046 Published Online October 2013 (http://www.scirp.org/journal/jbnb)
365
Effects of Precipitation Temperature on Nanoparticle
Surface Area and Antibacterial Behaviour of Mg(OH)2
and MgO Nanoparticles
Banele Vatsha1,2, Phumlani Tetyana1, Poslet Morgan Shumbula1, Jane Catherine Ngila2,
Lucky Mashudu Sikhwivhilu1, Richard Motlhaletsi Moutloali1
1Advanced Materials Division, DST/Mintek Nanotechnology Innovation Centre, Mintek, Randburg, South Africa; 2Department of
Applied Chemistry, University of Johannesburg, Doornfontein Campus, Johannesburg, South Africa.
Email: richardm@mintek.co.za
Received May 28th, 2013; revised June 29th, 2013; accepted July 15th, 2013
Copyright © 2013 Banele Vatsha et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
A series of MgO nanoparticles were prepared by first precipitating and isolating Mg(OH)2 nanoparticles from
Mg(NO3)2 at three different temperatures using NaOH followed by their thermal decomposition also at three tempera-
ture settings. The effects of temperature at which precipitation and thermal decomposition of the hydroxide occurred
were studied to assess their influence on nanoparticle size and surface area. The synthesised nanoparticles were charac-
terized using a suite of techniques including Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), Transmission
Electron Microscopy (TEM) and Scanning Electron Microscope (SEM) analysis. The average diameter range of MgO
nanoparticles ranged between 15 and 35 nm, while for the precursor Mg(OH)2 it varied between 28 and 45 nm. The
nanoparticle surface area obtained from BET studies was found in all cases to increase from 77 to 106.4 m2/g with in-
creasing temperature of precipitation. Antibacterial activities of the prepared Mg(OH)2 and MgO nanoparticles were
evaluated against the Gram-negative bacteria, Escherichia coli, and the Gram-positive bacteria, Staphylococcus aureus,
using agar diffusion method. A correlation between surface area and antibacterial activity supported the mechanism of
bacterial inactivation as the generation of reactive species. The Mg(OH)2 and MgO nanoparticles both exhibited pro-
nounced bactericidal activity towards the Gram positive bacteria than Gram negative bacteria as indicated by the extend
of the zone of inhibition around the nanoparticle.
Keywords: MgO; Nanoparticles; Precipitation; Crystallinity; Antibacterial
1. Introduction
The increased proliferation of infectious diseases due to
microorganisms found in medical devices, food packag-
ing, domestic appliances and water treatment systems has
elicited increased attention [1-3]. In the recent years, ex-
tensive efforts have been made to develop new or im-
prove known materials with antimicrobial properties.
Specifically, increased resistance of microorganisms to-
wards current biocides is of great concern specifically in
people of compromised immune systems like the elderly,
children and the sick. This has led to increased efforts to
explore new types of antimicrobial agents. In particular,
inorganic oxide nanomaterials like CaO, ZnO and MgO
have shown potential as effective alternatives in ad-
dressing some of these challenges [4-6]. For example, in
the last decade literature exploring the use of metal ox-
ides and specifically MgO has become widespread.
These studies revealed that indeed MgO is able to deac-
tivate both gram negative and gram positive bacteria and
several mechanisms of deactivation postulated [4,7]. Re-
lationship between particle size and activity was also
reported. This has led to our interest in Mg based materi-
als for applications in health and water [8].
Current research interest in inorganic nanomaterials
synthesis has focused more on controlling and tailor-
making materials into nanoparticles, nanorods, nanotubes,
nanoplates, etc. with the aim of streamlining their appli-
cations and efficiency [6,9]. Moreover, investigations to
develop versatile, cost effective and up-scalable methods
in achieving predictable nanoparticle properties are para-
mount to the success of introducing these materials for
widespread usage. These methods include sol-gel [10],
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Effects of Precipitation Temperature on Nanoparticle Surface Area and Antibacterial Behaviour
of Mg(OH)2 and MgO Nanoparticles
366
hydrothermal [11], laser vaporization, chemical gas phase
deposition and combustion aerosol synthesis [12]. How-
ever, in our investigations, there are drawbacks encoun-
tered from these conventional methods that tend to give
lower yield, poor control of shape and size and use of
corrosive chemicals. Indeed, it is generally known that
MgO’s unique properties are related to the size of the
nanoparticles [13]. Ouraipryvan et al. studied the synthe-
sis of crystalline MgO nanoparticle with mesoporous-
assembled structure via a surfactant-modified sol-gel
process [14]. The obtained results showed that the parti-
cle exhibited mesoporous structure and the diameter
range was approximately 35 to 50 nm. Continued search
for control of nanoparticle synthesis using simple meth-
ods is still relevant.
In this study the precipitation method was adopted
mainly because of its simplicity and relatively good con-
trol of the experimental conditions. The present study
was carried out to investigate the effect of precipitation
temperature on the morphology of MgO nanoparticles,
specifically the surface area, and its influence on anti-
bacterial (E. coli and S. aureus) activity. A strong corre-
lation between nanoparticle surface area and antibacterial
effects was observed giving an indirect way of establish-
ing mechanism of bacterial inactivation.
2. Experimental Section
2.1. Materials
Magnesium nitrate (Mg(NO3)2), sodium hydroxide (NaOH)
were purchased from Sigma Aldrich, South Africa. All
solutions were prepared using freshly prepared high
purity water from a Millipore unit, Q-POD. Escherichia
coli (E. coli) and Staphylococcus aureus (S. aureus)
strain were donated by Biolabels Unit at Mintek.
2.2. Characterisation Techniques
The XRD diffractograms were recorded on a Bruker D8
Advance X-ray diffractometer using a Co-Kα (1.7902Å)
monochromatic radiation source and a Ni filter with the
operating voltage and current maintained at 40 kV and 40
mA respectively in the 2
range of 5˚ - 80˚. The obtained
diffraction patterns were processed using Eva software.
Differential scanning calorimetric (DSC) analysis were
performed using air as oxidant at the heating rate of
10˚C·min 1 in a crimped aluminium crucible heated up to
a temperature of 500˚C using a Shimadzu DSC60 instru-
ment with a TA60WS thermal analyser and FC60A con-
troller. Scanning electron microspcopy (SEM) analyses
were done on a Nova NanoSEM 200 from FEI operating
at 10.0 KV. The Transmission Electron Microscopy
(TEM) images were acquired on a Philips CM120 Biot-
win Transmission Electron Microscope. The samples
were prepared by placing a drop of a dilute sample on a
carbon-coated copper grid (400 mesh, agar). The samples
were allowed to dry completely at room temperature.
Nitrogen adsorption measurements were performed at 77
K using a Micromeritics ASAP2010 system utilizing
Brunauer Emmett-Teller (BET) calculations for surface
area and BJH calculations for pore size distribution for
the desorption branch of the isotherm.
2.3. Synthesis and Characterisation of
Magnesium Oxide Nanoparticles
Mg(NO3)2 (0.2 M) and NaOH (0.4 M) solutions were
prepared using deionized water from Millipore water
purification system. Precipitation was induced by drop-
wise addition of NaOH into the Mg(NO3)2 solution under
continuous stirring for 1 hour at different reaction tem-
peratures (i.e. 23˚C, 60˚C and 85˚C) at pH 12. The re-
action mixture was cooled down to room temperature,
centrifuged and washed with copious amount of high
purity water and ethanol for effective removal of im-
purities. The final product was dried at 80˚C for 24 h and
calcined at different temperatures (i.e. 500˚C, 600˚C and
700˚C). Table 1 summarises the materials used and
reaction conditions.
2.4. Antibacterial Study Using Agar Diffusion
Method
Antibacterial activity was performed using agar diffusion
method adopted from Sundrarajan et al., [13]. Bacterial
cultures were grown overnight at 37˚C by adding a single
colony in 100 ml Luria Bertani Broth. E. coli and S.
aureus cultures (0.1 ml each) were plated out onto indi-
vidual Nutrient Agar plates using the Aseptic technique.
Holes/Wells were made on the nutrient agar inoculated
with bacteria and (about 0.5 mg) MgO nanoparticle
suspensions were decanted into the wells and the plates
Table 1. Precipitation and calcination temperatures used
for MgO nanoparticle synthesis.
Type Sample Precipitation
temperature (˚C)
Calcination
temperature (˚C)
1 23 500
A 2 23 600
3 23 700
4 60 500
B 5 60 600
6 60 700
7 85 500
C 8 85 600
9 85 700
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Effects of Precipitation Temperature on Nanoparticle Surface Area and Antibacterial Behaviour
of Mg(OH)2 and MgO Nanoparticles 367
incubated overnight at 37˚C. Finally, the diameters of
zones of inhibition around the wells were measured
against the control strain and measured with callipers.
Zone of inhibition is the area in which the bacterial
growth is stopped due to bacteriostatic effect of the com-
pound and it measures the inhibitory effect of compound
towards a particular microorganism.
3. Results and Discussion
3.1. Preparation of MgO from Mg(OH)2
Nanoparticles
The synthesis protocol employed for MgO nanoparticles
was a precipitation-calcination method. This involved
dropwise addition of sodium hydroxide into aqueous
solution of magnesium nitrate at various temperatures
between 25˚C to 85˚C to induce precipitation to form of
magnesium hydroxide. In order to finally obtain MgO
nanoparticles, the Mg(OH)2 powder was calcined at
500˚C, 600˚C and 700˚C through the decomposition of
Mg(OH)2 to MgO nanoparticles (Scheme 1).
Temperature of precipitation is one method of ma-
nipulating nanoparticle size and morphology. For exam-
ple, Yildirim and Duncan observed that the size of
spherical ZnO nanoparticles could be manipulated by the
choice of precipitation temperature (13.0 ± 1.9 nm at
25˚C and 9.0 ± 1.3 nm at 80˚C), which they attributed to
changes in the nature of adsorption events between ZnO
crystals and organic molecules used as surfactants [15].
Even though, no surfactants were used in the current
method, good control and manipulation of nanoparticle
morphologies were obtained by varying the precipitation
temperature. This indicates that precipitation temperature
might be an important factor in controlling morphologies
in instances of ionic effects rather surfactants are being
used. This will assist in lowering the overall cost associ-
ated with controlled nanoparticle synthesis. It is however
important to note that many methods and their variants
have been used to produce MgO nanoparticles [16-24].
3.2. Thermal Analysis of the Mg(OH)2
Transformation to MgO
Figure 1 shows the DSC and TGA profiles of the de-
composition and transformation of Mg(OH)2 nanoparti-
cles into MgO nanoparticles. The thermograms are do-
Scheme 1. MgO deri ved from Mg(NO3)2 precursor reaction
mechanism through the Mg(OH)2 intermediate.
Figure 1. TGA and DSC profiles of the decomposition of the
prepared Mg(OH)2 to MgO nanoparticles.
minated by one major thermal event centred on 380˚C
(DSC) with an onset at 250˚C (TGA) or 280˚C (DSC).
This onset marks the start of the decomposition of
Mg(OH)2 to MgO nanoparticles which occurs typically
during calcination of the hydroxide material [11,18,
25-27]. Other thermal events observed were in the tem-
perature regions around 100˚C, 240˚C and 465˚C. The
first event at 100˚C is probably related to the loss of
physically adsorbed or unbound water molecules on the
hydroxide. The thermal event around 240˚C is generally
attributed to the loss of chemisorbed water molecules
[28]. The thermal event at 465˚C that occurs after the
transformation of the hydroxide to the oxide is associated
with the phase transformation of amorphous MgO into its
cubic ordered phase [27]. This event, however, is not
widely reported or observed in literature during thermal
transformation of Mg(OH)2 to MgO.
3.3. X-Ray Characterisation of Mg(OH)2 and
MgO Nanoparticles
Figure 2 depicts the X-ray diffractogram of Mg(OH)2
nanoparticles precipitated at different temperatures, viz.
A at 23˚C, B at 60˚C and C at 85˚C. The three diffracto-
grams are similar and exhibit the same diffraction pattern
associated with Mg(OH)2 materials. The multiple dif-
fraction peaks from Mg(OH)2 at 22˚, 44˚, 60˚, 69˚, 74˚,
82˚ and 86˚ arise from the 001, 100, 011, 012, 110, 200
and 021 planes respectively according to a report by Sun-
drarajan et al. [13] and the XRD PDF2010: 000021169.
This confirmed that the formation of pure Mg(OH)2
nanoparticles is indifferent to the precipitation tempera-
ture used.
Figure 3 is the X-ray diffractograms of MgO nanopar-
ticles produced by calcining Mg(OH)2 at different tem-
peratures. The nanoparticles produced by calcination at
500˚C (A), 600˚C (B) and 700˚C (C) all exhibited iden-
tical diffraction patterns. The samples all had diffraction
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Effects of Precipitation Temperature on Nanoparticle Surface Area and Antibacterial Behaviour
of Mg(OH)2 and MgO Nanoparticles
Copyright © 2013 SciRes. JBNB
368
Figure 2. Comparison of X-ray diffract ograms of Mg(OH)2 nanoparticles precipitated at 23˚C (A), 60˚C (B) and 85˚C (C).
Figure 3. X-ray diffractograms of MgO particles prepared from Mg(OH)2 precipitated at 23˚C (A), 60˚C (B) and 85˚C (C).
peaks at 44˚, 50˚ and 74˚ are assigned to 111, 200 and
222 planes respectively (XRD PDF2010:000021207).
This clearly indicated that Mg(OH)2 was completely
transformed to crystalline MgO with no traces of impuri-
ties observed. The results also show that calcining at the
relatively low temperature of 500˚C is adequate to pro-
duce pure MgO using this method. The crystallite sizes
(Table 2) were also calculated on the samples using 111,
200 and 220 diffraction maxima from the half-width of
diffraction peaks using Scherrer’s equation. The calcu-
lated crystallite sizes are comparable (on the lower end)
with the sizes obtained from TEM analysis for MgO
nanoparticles. According to the XRD data, the mean
crystallite sizes of the MgO nanoparticles were calcu-
lated by using the Debye Scherrer formula.
0.9
cos
D
Effects of Precipitation Temperature on Nanoparticle Surface Area and Antibacterial Behaviour
of Mg(OH)2 and MgO Nanoparticles 369
Table 2. BET results of MgO nanoparticles calcined at
700˚C.
XRD crystallite sizes (nm)
Sample name
(precipitation
temperature)
Surface area
(m2/g) (111) (200) (220)
Mg(OH)2 (23˚C) 77.6 18.5 18.8 17.5
Mg(OH)2 (60˚C) 88.1 10.7 15.3 15.3
Mg(OH)2 (85˚C) 106.4 13.7 16.3 15.0
where λ = cobalt wavelength (1.7902Å), θ is the bragg
diffraction angle of the XRD peak, β is the measured
broadening of the diffraction line peak at an angle 2θ, at
half its maximum intensity (FWHM) in radian.
3.4. BET Analysis of Mg(OH)2 and MgO
Nanoparticles
The BET method was used to obtain specific surface area
for the MgO nanoparticles. BJH analysis was used to
determine the pore size distribution. This indicated that
MgO nanoparticles exhibit a bimodal pore distribution
which contains mesopores of between 1.7 - 5.2 nm and
larger mesopores with pore size averaging at 34.0 nm.
Rezaei et al. observed that the specific surface area of the
nanoparticles increased with increasing refluxing time
and temperature [29]. This is in line with our results that
showed that nanoparticles produced at different pre-
cipitation temperature showed that the specific surface
area increased with an increase in precipitation tem-
perature (Table 2).
3.5. TEM Analysis of Mg(OH)2 and MgO
Nanoparticles
TEM was used to analyse Mg(OH)2 nanoparticles pre-
cipitated at different temperatures. The general observa-
tion was that the shapes appear to be different (Figure 4).
It would be interesting to interrogate this observation
further but due to no access to a TEM instrument with
higher resolution, this idea was not explored further. The
average size distribution of the nanoparticles is shown in
Figure 4 with a mean of 37 nm for Mg(OH)2 precipitated
at 25˚C (Figure 4(a) ), 28 nm for those produced at 60˚C
(Figure 4(b)) and 45 nm for those precipitated at 85˚C
(Figure 4(c)). Literature shows that precipitation tem-
perature has an influence on the shapes and not size, with
flakes being observed when lower temperatures are used
[13,30,31]. Others have recently reported that there is a
correlation between precipitation temperature and final
particle size as long as the reaction is allowed to progress
for longer periods at lower temperatures to allow com-
plete reaction and other processes to take place [32]. This
aspect however widely reported, gives conflicting con-
clusions in literature to result in one predictive approach.
In the current work indications are that the size variance
does not show an obvious relationship with the precipita-
tion temperature used an observation that was also made
by Lv et al. [33].
MgO nanoparticles prepared by calcination of Mg(OH)2
materials at different temperatures, viz. 500, 600 and
700˚C were also characterised using TEM. The overall
average size range determined by TEM analysis results
ranged from 14 to 32 nm (Table 3). The average size of
nanoparticles obtained at 500 and 600˚C were compara-
ble irespective of the precipitation temperature used to
produce Mg(OH)2 particles with a narrow distribution
between 14 and 20 nm. On the other hand, nanoparticles
obtained at 700˚C were relatively larger (between 27 and
32 nm). Even though the general observation was that all
calcination temperatures resulted in average sizes of less
than 35 nm, the general appearance of the nanoparticles
seemed different. Sundrarajan et al. also observed the
effects temperature had on the morphology of MgO
nanoparticles where the initial flakes were transformed as
the calcination temperature was increased [13]. In so far
as the precipitation temperature was concerned, it was
observed that nanoparticles precipitated at higher tem-
perature (85˚C) exhibited more defined shapes compared
to those obtained at 23˚C and 60˚C, indicating that nano-
particle shape and size might be influenced by the pre-
cipitation temperature of the precursor Mg(OH)2 powder
was produced. Mageshwari and Sathyamoorthy used a
precipitation temperature of 100˚C to produce MgO na-
noparticles with a 5 nm average, an observation that in-
dicates that other parameters are more important at con-
trolling particle size than precipitation temperature [34].
3.6. Antibacterial Studies
The antibacterial effects of Mg(OH)2 and MgO nano-
particles were investigated using gram negative and gram
positive bacterial strains, E. coli and S. aureus respec-
tively, by the well diffusion agar method. The zones of
inhibition, the clearing zones around the wells without
visible bacterial growth, were measured from three se-
parate plates and are captured in Figure 5. These zones
indicate the antibacterial activity of Mg(OH)2 and MgO
since no bacterial growth is observed in those areas. The
antibacterial activity, from both Mg(OH)2 and MgO, was
higher in S. aureus than that observed in E. coli tests,
This can be attributed in part to the general observation
that gram positive bacterial strains are more susceptible
to antibacterial materials as compared to gram negative
strains due to differences in their cell wall structure [35].
On the other hand, MgO exhibited consistently higher
activity against S. aureus bacteria than Mg(OH)2 (Figure
5). This is in line with literature observation by Panacek
et al. [36] and Pan et al. [37] that smaller nanoparticles
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Effects of Precipitation Temperature on Nanoparticle Surface Area and Antibacterial Behaviour
of Mg(OH)2 and MgO Nanoparticles
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370
A
C
Figure 4. TEM micrographs of MgO nanoparticles calcined at 700˚C (C) from Mg(OH)2 precipitated 23˚C (A), 60˚C (B),
85˚C (C) and with their respective histograms generated from the TEM micrographs.
are more active than larger ones and thus MgO nano-
particles, which are smaller than Mg(OH)2 nanoparticles,
are more effective towards inhibition of bacterial growth.
The mechanism of bacterial inhibition for nanoparticle is
usually attributed to the generation of reactive radicals
[38]. This mechanism is also assumed in this study and
therefore the different antibacterial activities of the mate-
rials are related to their ability to generate such radicals,
Effects of Precipitation Temperature on Nanoparticle Surface Area and Antibacterial Behaviour
of Mg(OH)2 and MgO Nanoparticles 371
Figure 5. Antibacterial activities of Mg(OH)2 and MgO as measured by zone of inhibition method.
Table 3. Summary of preparation conditions and average
nanoparticle sizes (nm).
Precipitation
temperature
(˚C)
Calcination
temperature
(˚C)
500 600 700 Mg(OH)2
25 15 14 27 37
60 16 18 23 28
85 18 20 32 45
a property that is dependent on the surface area and
crystal edges exposed. It is known that the higher the
surface area per unit mass, the more reactive material
becomes. The density of radicals produced, and hence
the inhibition ability is directly related to this property
and hence the trends observed in this study (Table 4).
For example, MgO is known to be more reactive than its
hydroxide and hence the activity trend observed whereby
the oxide is more active than the analogous hydroxide in
all cases studied. Within the materials themselves, e.g.
oxides, the activity trend follows the surface area and
nanoparticle size trend established in prior sections.
4. Conclusion
Synthesis and antibacterial effects of Mg(OH)2 and MgO
nanoparticles were undertaken and factors contributing
to the formation of well-defined nanoparticles were
explored. Temperature at which calcination of Mg(OH)2
to MgO was performed was found to be more important
than the temperature at which Mg(OH)2 was synthesised
in determining the final MgO nanoparticle size. A simple
and cost effective method to obtain nanoparticles with
narrow size dispersion was established in this study. The
antibacterial activities of the prepared Mg(OH)2 and
MgO nanoparticles were studied using well-diffusion
method. Both these nanoparticle types exhibited greater
Table 4. Relationship of MgO (calcined at 700˚C) anti-
bacterial activity towards S. aureus and surface area.
Activity against
S. aureus
(
mm
)
Precipitation
temperature
Mg(OH)2MgO
Surface
area
MgO
MgO
(nm)
700˚C
Mg(OH)2
(nm)
23 6.5 8.6 77.6 27 37
60 6.8 7.0 88.1 23 28
85 7.5 10.2 104.2 32 45
antibacterial effects towards Gram-positive bacteria than
Gram-negative ones. In all cases, the antibacterial acti-
vity of Mg(OH)2 was lower than that of MgO, i.e. against
both E. coli and S. aureus. The antibacterial activity was
closely related to the surface area than to the actual nano-
particle size observed. The relatively low cost and abun-
dance of Mg based nanoparticles (Mg(OH)2 and MgO)
versus other metal based antibacterial agents make this
material a viable alternative for this application, i.e. E.
coli and S. aureus inhibition.
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